I wrote earlier in this thread about group rides, but I will expand just a little. My wife, now 64, decided two years ago she wanted to ride with me, some, which means a lot faster than she was riding. After rejecting group rides for a while, she finally attended a few. She was dropped right away on every ride.
However, she kept going back and was soon hanging on. These group rides were 14-15 MPH. Last year she announced she was getting "bored" with the slow rides and has stepped up to a higher speed group. She even leads some rides now.
My wife was never athletic before and questioned what I was doing out there on a bicycle. She now has two CF road bikes and a Surly Long Haul Trucker for our tour/camping rides.
Yesterday she pulled up next to me on her road bike with about 1 1/2 miles to the end of the ride. She went ahead at 20-21 MPH for that last 1 1/2 miles as I stayed on her wheel.
She attributes all her improvement to joining the group rides and not quitting when getting dropped time after time. She kept trying to catch up and kept trying to hang on. The speed improvement came with her desire to stay with the group.