In preparation for my first century attempt next week, my 17yr old son and I rode 78 miles yesterday with 5000 feet of climbing. Our Average speed was 15mph. The route has most of the hard climbs in the last 10 miles.
We had 2 15 minute stops and one half hour for lunch, plus a few 5 minute stops to work out where we were. We started out in 39F at 9am and the temps got up to the high 50's, A long sleeve merino wool jersey really worked well. Generally the ride went well, we had enough food and water and are both confident we can do 100 next week.
The basic ride is hee, we added a few miles to and from home, and getting lost and finding a dead end and having to go around it.