It isn't far fetched at all to do a 600 mile tour other than the expected daily mileage of 100. Lots of people have done the Trans America who were not previously avid cyclists and with minimal preparation. The thing that makes his trip less feasible isn't that it is long but more that it isn't long enough to start slow and build mileage as he goes. Still a reasonably fit young person should be able to manage 65 miles a day if they pack light and are determined enough (assuming reasonable terrain).

One example... One of my Trans America companions was never an avid cyclist. While she did own a bike she had not ridden it through her undergraduate years. She borrowed a bike from me and got in a few rides in the few weeks before the trip. Her longest ride was 33 miles and all of her rides were unloaded. By necessity she didn't get her bike for the tour until two days before we left. I spend some hours fitting her and setting up the bike before we had to box it up. It helped that she was a runner and an avid outdoors-person with good camping gear and skills. She did fine (we started with easier days) and finished the 4244 mile tour in 73 days.