Leg speed is, simply, the speed your legs will move without you trying to move them faster. You have a certain leg speed on a bike, and another one when running, or, eventually, walking. Because leg speed decreases over time in the same way that entropy increases, or because of it.

How many times have I been out on an epic ride, cracked like a robin’s egg dropped from the nest, and then fallen back to leg speed to get myself home? Lots of times is the answer. My companions, if they stay loyal, wondering how it is that we can be moving this slowly and still staying upright.

In the midst of a blood-sugar crash, I often calculate the distance home and think, “Yeah. I think I can get there on leg speed alone.” It’s quite a consolation. It means I can crawl away, lick my wounds, and fight again another day. Honestly, on the occasion this scenario materializes, I much prefer to be by myself. The pity escort just compounds the damage to ego.

In accumulating hundreds of running base miles this year, I have succeeded in increasing my bottom line leg speed. It’s like my legs know how to run ten-minute miles and don’t bother to involve me anymore than necessary. The only downside to that comes when I want to run eight-minute miles. It’s hard to be good at running fast when you’re so good at running slowly.

I rediscover leg speed on every long run, anything over 12 miles, let’s say. At that point, I’m managing on quick calorie bursts from gels and chews, and my body just settles into whatever rhythm it can find. I have yet to find the cracking point here, on up to 20 miles, and every time we reach the car when the run ends, I wonder how long I could have sustained that mode of running. I think about it a lot in fact.

In my mind’s eye, the ultramarathons to come are achieved mainly via leg speed, not running, but not not running, the lowest common denominator when fuel, strength and will are averaged out.

On the bike, the real compromiser of leg speed is incline. I can still feel the hurt in my soul from crawling up wet, gritty mountainsides in Vermont mud season. On two wheels, there is no equivalent to walking, unless it’s walking, and that is a step I just can’t take.