My intro to biking again was the Bike to the Bay here last year. 77 miles down and then you come back the next day. Pretty flat, but a few windy areas as you get close to the beach. For my first ride I went one way only and loved it. The ride was in early October and I started training in late May. Each weekened I rode about 16 miles each Sat and Sun and during the week snuck in a hour or so of easy riding once or twice. Two times during that summer I did longer group rides, about 35 miles. I only had a heavy hybrid style bike. The day of the event I did get tired a few times, but nothing major. I just took it slow and steady and stayed with my group of families and singles from our fellowship. So it seems to me that if you start training now and have a lighter bike, you should do fine and could do both ways, especially if it's broken into 2 days.
The fund raising was the harder part for me, and even that was OK. I had much fun doing the ride and training.