In good shape, on a flat course, on a nice day, 5 1/2 hours solo should be doable for many people. In a decent group just about anyone could do it - you just need to be sitting on the right wheel.

In April I did my first flat solo century of the year at 6:02. In May I did the same route solo again at 5:40. I was going to try to break 5 hours this year, but ended up doing two 30-45 mile fast hilly group rides a week until recently and cutting down on long distances. I did go out once in August and see how long I could hold 20 mph + - made 46 miles in a fair loop (to eliminate possible wind and elevation type advantages).