There's a bike path that runs around Lake Dillon. IIRC, you can make a loop around Lake Dillon, on th bike path in part, and the road over to Breckenridge.

Also you can go from Dillon to Frisco to Copper, up Vail Pass, and over to Vail all on bike path. The ride up Vail Pass parallels Interstate 70, but for the most part you don't even know the Interstate's there, and its very scenic.

It's a good sized climb, but not terribly steep. IIRC correctly Dillon to Vail and back is in the 80 mile range.

If you don't have the time for the full ride, you could ride to the top of Vail pass and turn around.