I commute in Berlin by bike. I'm reasonably fit and on an open road I can average about 28km/h for a couple of hours. In the city though my average speed is about 18km/h because of traffic lights. There are very few people who overtake me on my commute. I don't see how you would manage 20 miles (32km) in less than 90 minutes without running all the red lights and riding like these madmen here: https://www.youtube.com/watch?v=jhaGzAnEOd8