If you want something off the beaten path (so to speak), I've enjoyed riding Indian Brook Rd. from 9D south of Cold Spring up into Fahnestock Park, then across Dennytown Rd. to Sunken Mine Rd. That's about 8 miles, graded dirt for the most part, and lots of up and down. If you want to make it more interesting there's a trail that runs south of Indian Brook, and you can loop up to 301 for the return if you're in a hurry. Adding the run to and from Beacon on 9D and you're probably looking at ~30 miles.