This is how I look at it as well.

Particularly on hills. I can ride 6-7 hours easy at a decent (18-20mph on my recumbent, 22-24 in the velo) pace on level ground, but as soon as I start going into hills where higher output is needed, I poop out fast. I try to push myself, but it's a little easier if there's something to chase that I can see.

It also helps that I can usually catch even a fast rider once it levels out, at least in the velo.