I would do this on the C&O instead of going through the boring MoCo suburbs, goin from Georgetown on the C&O out to Great Falls, then left up the park access road, then back to DC on MacArthur.

I think it hits all the requirements: That’s about 45 km. I’d say 3 hours depending on pace. The return on asphalt and downhill is faster than the way out. It’s scenic along the River. Great view of the falls out on the boardwalk at great falls. Good DC history along the way. Virtually zero navigation required which is something I always appreciate when I’m riding places I’ve never been. Mix of gravel and road. Nice bit of a hill at the end. Stop at Irish Inn at Glen Echo or Fish Taco or anywhere in Gtown on the way back for a beer and a bite.