I think it is certainly doable. I mean I am 45yo and can average 20-21 by myself on a 15 mile loop around my office without too much trouble. I am certainly not a high level cyclist. I put in maybe 150-175 a week. Nothing too serious with training. I mean I do some variations on intervals and just have fun with it. I like to ride hills, try to max out, but more just to push myself.

We have a local 10mile time trial and I see some local guys in the 25mph range. My best was right at 22.1.