Why Running Harder After 50 Makes You Slower

Your easy days got too hard — and your body is paying for it

You added miles. You added a hard day. For a few weeks it worked. Then your body started going backward — and your age isn’t the reason why.

I’m 73 years old. I’ve run more than 50 ultramarathons, I’ve won age-group awards into my late 60s and 70s, and I’m a two-time heart attack survivor. I want to be clear about something: I’m a mid-packer, not an elite. I’ve never had elite genes or elite talent.

After my second heart attack at 62, I tore my whole approach to training down to the studs and rebuilt it. Everyone expected me to slow down and fade out. Instead my running got better. I got faster at an age when the charts say you’re supposed to be finished.

I’ve spent the last eight years coaching runners in their 50s, 60s, and 70s through the exact problem I’m about to describe. I’ve watched it play out hundreds of times, and it almost always starts the same way.

Here’s the pattern. Around 50, you slow down a little. The pace that used to feel easy starts to feel like work, so you do the thing that worked your whole life — you push harder. You add miles. You add a hard day. Your easy runs creep up into medium-hard runs, because backing off feels like quitting. For a few weeks it works. Then it stops. You plateau, you start going backward, you’re tired all the time, and your knee or your calf starts to ache.

Pushing harder didn’t fail you because you’re old. It failed you because after 50 it’s the wrong tool. Here are three reasons running harder makes an older runner slower, heavier, and more injured — and what to do instead.

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Harder running starves the engine that’s supposed to carry you

Your body makes energy two ways. Your aerobic system runs on oxygen and burns mostly fat. It’s slow, steady, and you can run on it all day. Your anaerobic system burns sugar fast and dumps a lot of fatigue and stress into your body in the process.

At 30, you could lean on that sugar-burning system every day and bounce back by morning. After 50, two things change. You recover from hard efforts more slowly. And your aerobic system — the fat-burning one — is the system that keeps getting stronger for decades when you actually train it.¹

When you answer your slowdown by pushing harder, you feed the sugar-burning side and you train the fat-burning side less. You end up with more fatigue and less of the staying power that keeps you running for life. It feels productive. Your body is quietly going the wrong direction.

The main culprit has a name: junk miles. Junk miles are runs that are too hard to recover from and too easy to build real fitness. Coaches call it the gray zone — that medium-hard effort where you’re breathing harder than a conversation but nowhere near a true hard workout. My mentor on low heart rate training, Dr. Phil Maffetone, puts it plainly: most athletes “run often in that gray zone, not slow enough on the slow days and sometimes not hard enough or too hard even on the hard days.” His warning is blunt — “when we don’t train the aerobic system, we lose aerobic function. That aerobic system becomes literally deficient.”²

Run that way for years and you build what I call aerobic deficiency syndrome (ADS). Your aerobic system ends up underbuilt, so you burn through sugar and run out of gas early. Most runners our age live in the gray zone. Nothing is truly easy, nothing is truly hard, and everything sits in the soggy middle.

If this is landing for you, hit reply and tell me the age when running first started to feel harder. I read every one, and I’m curious how many of you say it happened right around 50.

Harder running teaches your body to store fat, not burn it

This one doesn’t just show up on the clock. It shows up around your middle.

Your aerobic system is your fat-burning system. Spend all your training in the gray zone and your body gets worse at burning fat and better at relying on sugar. Researchers measuring fat and carbohydrate burning in endurance athletes found that the best-trained aerobic engines burn dramatically more fat at the same effort — that’s a trainable skill, not a fixed trait.³

There’s a hormonal cost too. Constant training stress keeps your stress hormones high, and over months elevated cortisol pushes your body to hold onto fat, especially around the belly. In one study, lean women who secreted more cortisol under repeated stress carried more abdominal fat than women who didn’t.⁴ You run more, you feel more tired, and the scale climbs anyway.

My mentor Mark Sisson of Primal Health has been saying it for years: you can’t outrun a body that’s been trained to depend on sugar. This is a fuel problem, and a fuel problem is fixable. Build the aerobic engine and you turn back into a fat-burner — on the run and at rest.

Harder running breaks tissue down faster than you can rebuild it

This is where training like you’re 35 hurts the most. After 50, your tendons get less forgiving, you lose muscle, and your tissues repair more slowly than they used to. Stack hard runs back to back with no easy aerobic running in between, and the damage piles up faster than your body can rebuild. One day the soreness becomes a real injury that costs you weeks.

Then the spiral starts. You get hurt, you rest, you come back feeling behind, so you push to catch up — and that hands you the next injury. Round and round it goes. After 50, the fastest way to lose your fitness is to get hurt. Almost every injured runner I coach was running too hard on their easy days and too easy on the one day they should have pushed.

The decline researchers measure in older athletes isn’t mostly about aging muscle — it’s driven in large part by the drop in training quality that happens when fatigue and injury chip away at what you can actually do week to week.⁵ Protect your easy days and you protect your training. Protect your training and you protect everything.

What training with your age looks like

Here’s the whole fix, and it’s simpler than the gray-zone grind you’re doing now.

Most of your running should be genuinely easy. Slow, conversational, at a heart rate low enough to build your aerobic system without beating up your recovery. For a lot of older runners that lands right around 130 beats per minute — slow enough that walkers might pass you, and that’s fine. Use your Garmin to hold that ceiling honestly, because your legs will lie to you and the watch won’t.

Then one small slice of your week is truly hard, with a clear purpose. Your easy days stay easy so your hard days can actually be hard. That mix — mostly easy, a little truly hard — is what makes an older runner faster and more durable.⁵

Two quick tests to see if the gray zone has caught up with you:

1. The 130 check. On your next easy run, watch your Garmin. If you can’t hold a conversation at 130 bpm, your aerobic base needs work.

2. The waist check. Measure your waist. A waistline creeping up while your mileage holds steady is your body telling you it’s burning the wrong fuel.

The payoff that makes the slow weeks worth it

When you build your aerobic system instead of grinding the gray zone, your slowdown starts to reverse on its own. You get better at burning fat, so your endurance climbs and your waistline comes back down. You step off the injury treadmill, because your body finally gets the recovery it’s been begging for. And your easy pace gets faster while your effort stays exactly the same.

I’ve watched runners in their 60s shave minutes per mile doing less hard running, not more. That’s not a trick. That’s what an aerobic engine does when you finally feed it.

Picture next spring. Same easy effort, but the pace on your watch is faster. Your waistband has loosened a notch. Your knee stopped barking weeks ago, and you’re not counting down to your next forced rest. That runner isn’t a younger version of you. That’s you, training with your age instead of against it.

I’m 73 and I’m getting faster. You’re not done slowing down because you’re old — you’re slowing down because you’re grinding. Fix the grind, and you’re not done yet.

Are you training too hard on the days you’re supposed to go easy — and too easy on the one day you should push?

What age were you when running first started to feel harder? Hit reply and tell me. I read every one.

Have a great week!
-Herb
”Run Smarter, Live Longer”

P.S. Ready to go deeper?
The Forever Runner Method is a proven, step-by-step framework built specifically for runners over 50 who are tired of getting hurt and are ready to run smarter. If you want to discover how, Grab a copy of my book at Foreverrunner.com.

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Notes

1. Hirofumi Tanaka and Douglas R. Seals, “Endurance Exercise Performance in Masters Athletes: Age-Associated Changes and Underlying Physiological Mechanisms,” The Journal of Physiology 586, no. 1 (2008): 55–63.

2. Phil Maffetone, “Your Performance Is Dropping? Here’s Why,” interview, accessed via the Forever Runner research library, 2025.

3. Iñigo San Millán and George A. Brooks, “Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals,” Sports Medicine 48, no. 2 (2018): 467–479.

4. Elissa S. Epel et al., “Stress and Body Shape: Stress-Induced Cortisol Secretion Is Consistently Greater Among Women with Central Fat,” Psychosomatic Medicine 62, no. 5 (2000): 623–632.

5. Stephen Seiler, “What Is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes?,” International Journal of Sports Physiology and Performance 5, no. 3 (2010): 276–291.