Don't know how accurate this is, but http://bikecalculator.com/ tells me that about 200 watts should be enough to reach 20 mph on the default settings. According to https://www.cyclinganalytics.com/blog/2018/06/how-does-your-cycling-power-output-compare 200 watt ftp is only the 12th percentile for males in the sample. For reference, I'm just a nobody with a 19-minute 5k and I managed 210 watts for an hour. And the only times I bike are a few times a year as leisure group activities with other non-bikers. An actual athlete should have no problem being better than me, so I'd say he could maintain 20 mph till boredom.