You'll be fine on 28s.
When are you going? I could go for some C&O.
I've been bike-lazy for the last 2 months with lots of stuff going on at work (just changed jobs) and now I'm moving at the end of the month so I've got to pack.
If you want to go ride a section together sometime I'm game. I live in Laurel but my office is in Rockville/Shady Grove only about 4 miles from Swains Lock in Potomac. I believe this is about mile 15-ish on the trail. I certainly have non-fg bikes that are more than capable of riding the trail I just thought it might be fun to do it fixed since it's pretty flat.