I have done many, many organized rides. I have a riding partner that usually does them with me. We get excited at the start of the ride at the thought of riding in a big group. We imagine ourselves in the peloton of LeTour. About 5-10 miles into the ride, the fast guys have dropped us and we've dropped the slow guys, and it's just us again. The only time we run into other riders is at the rest stops.

One word of caution, however. Going from 50 to 100 miles is not exactly just doubling your distance. To me, it's more like 4x as hard. I had done many metric centuries (63 miles, some with over 7000 feet of climbing) before doing my first century. But, adding another 40 miles onto the end of that ride was a big deal. Maybe it was mental as well as physical. I've talked to a lot of people and it seems like there's a big mental hit that comes around mile 80-90. You realize how far you've come, but that you still have a ways to go. I bonked at about mile 92 of my first century. Went out too fast and too hard.

I'm not saying that to be discouraging. I'm saying that so you don't make the same mistakes I made. Deliberately ride the first half a mph or two slower than you know you can. Then you'll have plenty of reserve for the second half.