You can hide being unfit on a road bike pretty well, hard to do on a mtn. bike unless you bike in your backyard or the parking lot.
Nah, I'd say in many cases it's just the opposite.
If you're a skilled mountain biker, you can save a lot of time in the technical parts of a rough MTB trail, and as soon as you hit the flats, your lack of fitness will become apparent.
I've never been someone to have great biking endurance, but it was best illustrated the time I entered a mountain bike race and turned out to be switching spots in the race with an ex-road biker.
On the rough difficult trails, I left him in the dust. By the time I exited the forest, I was way, way ahead of him and couldn't see him anywhere. Then we had this long stretch of flats and road and I saw a speck slowly gaining on me. By the time the we had finished that stretch, he was way ahead of me. And then when we got back onto the rough trails I caught and passed him again.
Yeah, I might have had a bit more strength than he did for cranking over the anaerobic parts, but the main reason is because he was more hesitant on the technical parts and thus a lot slower. Overall though I'd say he was in much better shape than me, but I was a better technical rider.
Unfortunately, now I'm just bad at both cuz I sit at a desk all day.
