You're in for a treat. I've been in Arlington for 6 months and have found the bike network to be excellent. You're across the road from the Mt. Vernon trail and you'll be able to get to the W&OD trail easily.

Check out the Arlington Bike Loop. Almost all of it is in on paved bike/walk paths with only a few surface crossings where cars are involved. The area has good bike lanes on surface streets, too. The whole region is well connected with bike lanes and paths.

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I couldn't tell you about riding at night, though. There are sections of the Arlington loop where it can feel a little creepy even in the daytime.

The only downside with COVID is that the paths are even more crowded than they usually are. I've just returned to the area in the past week after being away 2 months and haven't ridden yet. If I do, I'm going to try for the morning instead of the afternoon. There seem to be fewer people out in the mornings.