Most dads run their cardio at exactly one speed: hard enough to feel like it counted, not hard enough to actually change anything. Zone 2 cardio — the pace where you could still hold a conversation — feels too easy to matter. The research says that “too easy” pace is where the engine actually gets built.
One finding. One mechanism. One action. Let’s go.
The Finding
Sports scientist Stephen Seiler spent years pulling training logs from elite endurance athletes — rowers, cyclists, cross-country skiers, distance runners. The pattern held across every sport: roughly 80 percent of their training time was spent at low intensity. Easy. Conversational. The kind of pace most gym guys would be embarrassed to post.
And it’s not just an elite quirk. A 2014 study in Frontiers in Physiology took trained recreational athletes and compared training plans head to head. The polarized approach — mostly easy work with a small dose of genuinely hard intervals — beat the “medium-hard every session” approach for improving endurance. The people going slower most of the time got faster.
The athletes with the best engines on the planet do the least amount of medium. That should tell you something about your lunchtime tempo runs.
The Mechanism
Here’s the WHY. At a true Zone 2 effort, your slow-twitch muscle fibers are doing the work, and they’re running primarily on fat, burned inside your mitochondria — the power plants in your cells. Hold that effort for 30 to 45 minutes and you send a clean, repeatable signal: build more mitochondria, build better ones, get more efficient at clearing lactate before it piles up.
Push into the middle zone — that “feels like a workout” pace — and the signal gets muddy. Lactate production starts outrunning clearance, faster-twitch fibers take over, and the session now costs real recovery. But it’s still not hard enough to force the top-end adaptations that true intervals produce. You’ve paid the fatigue bill of a hard day and collected the adaptation of neither. Coaches call it the grey zone for a reason: it’s where cardio goes to be mediocre.
Zone 2 is cheap. That’s the whole point. It builds the base without draining the recovery budget you need for lifting, intervals, and Saturday morning with the kids.
The Action
One move: two Zone 2 sessions a week, 30 to 45 minutes each. Bike, brisk incline walk, rower, easy run — whatever you’ll repeat.
The pace test is simple: you can speak full sentences, not just words. If you’re using a heart rate monitor, 180 minus your age puts you in the neighborhood. It will feel too easy. It’s supposed to. Going slower on easy days is what lets the hard days actually be hard.
Log the Base Miles
Easy work only pays if it’s consistent, and consistency is a tracking problem. Log both weekly sessions in the Dadzilla App under Cardio Session Logging and note your pace at the same heart rate. When the same effort starts moving you faster, that’s your new engine showing up.
You’re not training for a mirror. You’re building the aerobic base that carries backpacks, coaches Saturday games, and never asks for a rest day.
This week’s rep: schedule two easy sessions — and have the discipline to keep them easy.
Ready to build the engine? Cardio Session Logging lives in the Dadzilla App.
Join the Dadzilla Beta →