Quote Originally Posted by OakLeaf View Post

So then the cyclist decides to go running. But our running muscles just aren't in the same kind of shape as our cycling muscles, besides the fact that even slow running is pretty intense cardiovascularly (whereas on a bike, if you want you can just take the pace down and pretty much cruise forever, long after you feel like your muscles are done).

So we're reaching our running lactate threshold at a HR that doesn't even begin to challenge a cardiovascular system that's in pretty good shape from cycling.
Everyone's different, but I have the opposite problem when I'm in good biking shape, but then start running again after not doing it regularly for some time.

My legs will be capable of maintaining a faster pace than my heart and lungs will permit, because running at a moderate/high intensity is more cardiovascularly challenging than cycling at the same intensity. Typically I'll have a lot of muscular endurance from cycling and I'm used to keeping a high cadence on my bike, so my running turnover is quick, which increases speed. But at a certain point my hr just goes through the roof because I'm not in cardiovascular condition to maintain the pace I'm running, and I have to slow down, even though my legs are not feeling heavy at all.

It takes me a few weeks of regular running to get back into cardiovascular condition to maintain a challenging pace, evenly, over distance. Some of it, also, is that you have to develop a sense of pace for running, and unfortunately having a good sense of pace for cycling doesn't necessarily transfer to running, you sort of have to get the feel back.

But the other thing you mentioned, about thinking about running workouts differently than cycling workouts, that never occurred to me because I was a runner before I was a cyclist, but it's definitely true. Running burns more calories per distance or per time period than cycling, thus it is comparatively a more intense workout. Think about it: cycling 26 miles is no big deal, right? but how many of us could get up tomorrow and run 26 miles without training? not me!!! And up to a certain point, you can build cycling mileage pretty quickly -- you can go from riding 10 miles to riding 20 miles, or 20 miles to 40 miles, or 30 to 60, without too much trouble (up to a point). But that's much less the case with running I think. Once you get much past 3 miles you can't really reasonably double your distance all at once anymore. It takes a long time to build up the endurance to run for two or three (or more) hours -- much longer than it takes to build the endurance to ride for the same amount of time. Even though the same training principles apply to both, you really need to think about them differently when considering how fast to build.