I live in Reston 50 feet off the W&OD. I agree that you will be unable to do certain types of training, but I regularly ride north from there and I'm able to do some decent intervals. No 20 minute, and 10 minute intervals are difficult but possible. Without the trail, I would get about 1/2 as much riding in. Living that close to it is a blessing. However, I agree that going south towards DC it will get crowded (lots of intersections too) and besides working on very high intensity 1-5 minutes intervals and LSD you will be SOL in terms of ability to train on the commute.