I’m not sure what the question is. Are you asking how much power it takes to do 20mph on a standard road bike? I’m assuming no wind and flat ground, right?
Looking at some Strava segments where I averaged 20mph, I would say 225 +/- 10w. This is on a Tarmac with 25mm training wheels and tires. Note that weight of the rider doesn’t make much difference on a steady interval on a flat road, so yours might be a little higher but not by much.