I can keep up very well in a 25-45 mile ride (with no stopping) at about 17 mph without drafting. My training route (30 miles, flat) doesn't include hills, but I can hang pretty good. If that fits with anything you were planning to do, it might be a good ride for me. I may even have a friend come along depending on the pace (he is more in the 15 mph pace (without drafting).
I know that with group rides, the pace could be about 19 or 20, and feel like about 17. So, I guess it's relative.
Edit: I am doing the HHH 100 mi. this year, so I would be up for a challenge as well, even though I might be whipped at the end (that's what it is about, right?).