A link to an alternate half century loop from Thurmont Community Park. This one goes counterclockwise, and after descending Hamburg Rd, a technical descent of several miles, is fairly flat for the last 12 miles or so back into town. There are refueling points in Smithsburg after about 15 miles and Meyersville after about 28 miles. The route climbs three distinctive mountain passes: up MD-77 at the start, around Greenbrier State Park on Keadle Rd, and over the Gambrill Park ridge on Crow Rock and Highland School Roads. However, all of the other "rolling" roads out here always have a tendency to be more hilly than I remember them.
Here is a 40 mile version that avoids the second mountain ridge for a little less mileage and a lot less climbing. It's probably more fun, to be honest.