I’ll cop to having suffered through a 4 hour trainer ride this winter, but once things get warmer/dryer, no way am I going to miss out on getting outside. There’s just nothing like getting used to the winds, hills, potholes, drivers, and other riders while cruising in your aerobars!
Now, as one who has lived in Shirlington in a townhouse and then a house backing up to the W&OD Trail, I’ll second it as a solid option for reasonably safe, long rides. The Shirlington end is definitely a bit more crowded than the west-of-Reston section, but it’s still quite manageable in a small group or solo. Once it ends in Purcellville (the west end), it’s also easy to head out into the country and cruise by the vineyards. Only downside to the W&OD is that it’s rather flat — it was once a railroad — so not the best practice for a hilly race.
Another great option for those long days ahead of you is to use the W&OD on one side of the Potomac, and cross over the river up at White’s Ferry (just a few bucks ferry cost), which is just north of Leesburg, and then ride to Poolesville and back to the DC area via River Road or other backroads out in that direction. That makes for a solid 80+ miles, and the River Road section feels far hillier if you do it last 🙂 I’ve also found that you can tack on a loop from Poolesville out to Sugarloaf and back. Once you’ve done all that, you’re in the 100 mile-ish range.
I’d also encourage anyone who hasn’t yet taken a day to head out to Skyline Drive to do so at least once this summer. It’s very hilly, but can be a spectacularly beautiful ride with minimal traffic. Its bonus is that it is a bit higher in altitude so it tends to be about 10-15 degrees cooler than the DC area — perfect for summertime (but remember to take cash for the entry fee and a blinky rear light — the park service requires it)