Two weeks ago, I finished a 75 miler on flat roads, only problem was the heat (94 w high humidity) which you won't have to deal with. If you did 72 miles on hills, a flat ride will seem very easy. I was surprised at my average speed of 17.2 MPH after having a lousy last 15 miles. Still, you should stay hydrated and take whatever your favorite anti-cramping product is. I think the biggest problem with a long, solo ride is the boredom. The suggestion to try and find some partners is a good one.