hi justa,
Rolling hills can vary, but basically it suggests something other than a half-mile or more consistent climb. It can vary from short stuff you can power over, to stuff where it pays to gear down and spin. Comparable stuff on the routes you mentioned that I’m familiar with:
Mt Vernon: think of going over the two bridges near the airport over and over, some a little longer, some a little shorter, but about that steep. Similar to the hills right at the end of the Mt Vernon Trail as you get up to George’s house
Custis: like the hills up from the river including the steepies along I66
Off W&OD, like going up Walter Reed. That would probably be about the steepest, although you will hit some that are longer
If you’ve done the complete W&OD there are some easily-accessible routes right off the trail that will give you some idea, like Simpson’s Circle & Meadowlark Drive in Paeonian Springs. Off of Dry Mill Road you can ride up Woodburn. That hill is about the worst you would run into on the Reston Century I think. You can ride the loop up that and then back into Hamilton on Harmony Church. In fact I think the Reston Century uses that route. And of course Snickersville Pike pretty much is the definition of “rollers” around this area.
I’ve done the RC twice, but skipped it last year because I’ve ridden all those roads so many times I got a bit sick of them. But it’s a great ride.
If you can you might want to try a flatter century or two first. The Cap2Cap ride is coming up in two weeks. That’s easy and scenic.
Have fun, and I’m sure you can do it!