[MENTION=364838]lakhotason[/MENTION]: Good point, I forgot to mention that I am into backpacking and long-distance walking in general (after 3 years of bike training and touring I'm sure I'm nowhere near the mileage I racked up on foot, just in the city, having never used a car or public transport for the better part of the last decade). I know for a fact that when it comes to walking I can't go below 3.75 mph on the flats when I've over 1 hour of walking ahead of me, because otherwise I'd get aches in my knees and hips and I'd start feel tired. I'm less constrained when the terrain has some incline and surface variation, probably because the range of motion in the legs is greater and the effort varies. For all my experience on foot, though, I'm less than impressed with how I managed to carry it over to bike touring, because I still don't have a proper feel for how I'm using the available energy on the bike.
[MENTION=117975]sstorkel[/MENTION]: Nice one, I was somewhat aware of muscular and cardiovascular endurance, or more specifically, reaching each one's limits, but I haven't tried to actively balance them, I'd just try to avoid running out of breath and reaching that burning sensation in the legs. It opens up another possibility in my particular case.

Riding a crank-forward, I normally have good control of cadence, because I can't stand on the pedals to accelerate. I have to upshift 4 times to get up to 20mph from a standstill on the crank-forward tourer, whereas I only shift twice, or even once if I feel my knees are alright, on the diamond-frame commuter. Thus spinning became second nature, and now that I think of it, I haven't reached the limits of my muscular endurance on the tourer too many times, but I run out of breath at least once on every ride. Last year, however, I was experimenting with 152mm cranks (down from 175), and I had to spin even more. I also had an aching knee, so perhaps I spun myself out while the short cranks prevented me from advancing at a reasonable speed.