I do almost all of my long training rides by myself. I usually can't find anyone who wants to ride the distance I want on the same day at the same time.

My longest training rides these days are approx. 70 miles. I have a 24-mile route that is a modification of a weekly ride that my LBS sponsors. The location where it begins and ends has lots of free parking, plus a decently clean public bathroom and a water fountain. I load up a cooler with all the drinks I will need for the entire ride, plus snacks and drinks for post-ride. So my car is the "rest stop."

I start early in the morning and load up myself and my bike with all the supplies I would bring if I were doing a regular metric century (i.e, no returning to the car until the ride is finished) -- 2 full water bottles, 8 packets of gu, etc. Then I go out and I ride the 24-mile ride loop for the first time. Back at my car for the first reststop, I refill my water bottles, then I hit the restroom and go out for the 2nd loop. Back at the car again to refill the water bottles again, stop at the restroom and then do the loop for a 3rd time. That makes a total ride of 72 miles.

I carry a spare tube, patch kit and pump on my rides in case of a flat tire. I have the phone numbers for several local cab companies written on the cue sheet in case I have any serious mechanical problems or can't ride back to the start for any reason. So far (knock wood) I haven't had to call them. I note the location of useful places like grocery stores and bike shops on my cue sheet in case I need to stop anywhere. I also note short cuts back to the start if I can't do the full 24-miles for some reason.

For other rides I do by myself, such as a hilly 30-mile ride that I can begin from home, I make sure I know the area beforehand. At the very least, I study google maps to familarize myself with the route, and I bring maps with me as a back up. I stick with residential areas so there will be people around if case of emergency. And I make sure there will be adequate places for refilling water bottles, finding bathrooms, etc.

So far it's worked out pretty well.