Slow down!! 20 miles in 48 mins translates into 25 mph. 36 mins is 33.3 mph. Average a more realistic 19-20 and you should be fine. My commute's 16.5 miles w/a 500 ft elevation drop on the way to work. 48-52 mins are my average times. The homeward leg takes about 10 mins longer because of the climbing. It's a marathon not a sprint.

Lots of water, brown rice, oatmeal, kale, fresh vegetables, fish, chicken, a protein supplement and sleep. When doing these types of distances it's necessary to treat oneself like an athlete. Can't over emphasize the importance of getting enough sleep.

If one is feeling burnout after only a few days adjustments need to be made.