I live in the alternate universe that is Virginia but I second the idea to time the return ride home so you know how long it takes. I did not think it was that far from Bethesda to DC. I wll say that compared with the CCT trail or biking from DC south to Alexandria, biking north along the Custis and W&OD trail is much more challenging physically, at least based on the conversations I have had with others, so count yourself fortunate in that regard.
I have noticed that Metro is not very picky about people getting on with bikes a few minutes before the curfews end. I’ve seen people try at like 6:45 or 6:40 pm and not get turned away. There is a sweet spot in time when most people have gone home for the day and the trains are still running frequently enough that they are not too crowded, usually around 6:40 to 7:15 or so in my experience.
I’ve also left my bike at work when there are serious thunderstorms.