Riding a decent road bike on flat terrain with no headwind, - 21 mph isn't difficult (not sure about comfy). Even a small headwind can change the equation for me though and I'll have to work hard to stay over 20 for an extended period of time. By the same token a bit of a tailwind can make 25 seem easy.

It's not hard to imagine somebody who's a bit stronger and in better shape than myself cruising along effortlessly at 21 even with headwinds and small changes in grade.