It’s roundabout, it’s stupid, and it has some off-road sections, but here’s how I’d go. http://ridewithgps.com/routes/2504953
You don’t actually have to ride on Greenbelt Road. Although it doesn’t show up on this map, there’s a dirt path from Branchville Ave to the Parking Lot at Chef’s Secret, and then you can follow around the back parking lot to Cherrywood lane to go left.
And there’s a sidewalk back under the beltway from Lakecrest Drive to the office park.
Traffic sewer hell out there — better to avoid, in my opinion and just go the long way around.
That said, if you go in the back entrance to Greenbelt National Park, that’s pretty good too, but like Ron said, some big hills. I’ve never tried the dirt trail/Pontiac Street route, that might be best…
-Jeff