If your “easy” runs don’t feel so easy anymore, you’re not imagining it. As your aerobic system gets stronger, the same heart rate can suddenly feel like more work. That shift can make even disciplined runners second-guess their training, but we’re here to explain why it’s actually a good sign.
The pace problem
In a recent Instagram post, Canadian endurance coach and elite athlete Kylee Toth explained: “As your aerobic system improves, the pace or power you can hold at Zone 2 goes way up. What used to be a jog at [a heart rate of] 140 bpm [beats per minute] might now be a brisk run or close to race pace. Same heart rate, but higher speed, impact and muscular stress.”
She adds, “The ‘zone’ doesn’t change, but the strain on your body and the recovery cost both increase.” That shift doesn’t happen overnight. It takes a steady diet of Zone 2 running—often months of consistent aerobic work—before you reach the point where “easy” has quietly turned into faster, heavier running.
Progress disguised as fatigue
“Metabolically, training adaptations like greater mitochondrial density, improved fat oxidation and more efficient lactate clearance are happening,” Toth explains. Those upgrades let you go faster at the same heart rate, but they also load your muscles and joints more. Your cardiovascular system adapts quickly, but the rest of the body takes longer to adjust. That mismatch is what can make steady runs feel harder even when you’re fitter.
Of course, not every tough day means progress. Heat, poor sleep, dehydration, hormonal changes or stress can all push heart rate and effort higher. If those are ruled out and Zone 2 still feels demanding, it’s likely a sign your aerobic system has leveled up. Sports scientists call this the “performance paradox” of aerobic development: the more efficient you get, the more recovery you need. Your cardiovascular system may be calm, but your musculoskeletal system is working overtime to keep up.
How to manage it
Re-test your zones regularly, since thresholds tend to drift upward as your fitness improves. Recalibrating your heart rate or pace zones every few months helps keep easy runs truly easy. If Zone 2 pace feels forced, slow down—keep effort easy even if that means ignoring heart rate for a while. It also pays to be intentional about recovery: if fatigue lingers, back off the pace or cut mileage until your legs feel fresh again. Track how you feel, not just data. Perceived effort often tells you more than numbers when zones shift.
