Running a flat 5k (not a hill climb) predominantly involves your pulling and lifting muscles (hamstrings and hip flexors, respectively..calves and ankles doing much of the pushing), riding always requires "pushing muscles" predominantly (quadriceps, while calves and ankles minimally involved).

I am a **** runner (7:30 pace for 5k, best 1 mile = 6:15), but I've had some success at rowing (all pushing with the lower body, pulling with the upper body) and would not have a hard time averaging 22+mph for 15 flat miles on the bike. I've also spent minimal time running, but having trained for so long for rowing, it lends itself more toward cycling than running.