Most all of my riding is solo including centuries and the 60-80 milers leading up to them. All of the organized rides I do are solo. All you need is a watch and your route and you can compete with yourself. I have a blast on the centuries competing against time goals for the ride.

If you plan well, unsupported long rides are easy to set up and attempt. Route yourself an initial loop, say 30-50 miles out and back to your initial starting point. Your starting point then becomes your rest station/ending point also. Use it to refill bottles and access snacks/fuel. After that plan 10-20 mile loops that go around your starting point/rest station. If you plan them right, you will never stray more than 3-5 miles from your starting point. So if you bonk, or have problems you can get back to your starting point easily or help is close by. You can go as far as you choose and cut off your ride when you need to. If you are really trying to find your limit, route yourself a "Champs Elyesse" around your starting point/resting station. Then when you start to tire, move to it and just keep doing the loop until you bonk and troll/coast back to the rest station. On these rides my house is the rest station/ starting point.