I do have to say though that I rode my son's trainer last night and was worn out trying to keep even 75 as my cadence while in a low gear and almost no resistance. He has one of his old 29er's on it that has 170 mm cranks. Yes I agree not much, but that extra 5 mm up and 5 mm down of leg travel wore me out and I felt like I was thrashing right from the start.

On my bike in the real world with 165 mm cranks, I find myself sometimes cruising at over 100 cadence and not thinking anything of it. And perfectly comfortable. Similarly, my wife's stationary bike that I use sometimes has 160 mm cranks and I can stay on it as long as I can my bike.

May son's trainer, I was worn out after 30 minutes. Not sure if that's a good type of getting worn out or a bad type of getting worn out. The only obvious difference so far is the crank length. But I'm going to try and look into it more as I get a chance.