I once rode with a guy who had recently come fourth in our national 24-hour TT championships. He had covered over 460 miles in the 24 hours. I asked him how long he had spent off the bike in that time, and he told me it was about fifteen minutes in total. So 500 miles without stopping except for natural breaks is perfectly feasible in the right terrain, if you have your nutrition and hydration needs sorted out. I must say I don't fancy it myself, though, extreme distances aren't really my scene.