Big question: do you have the South Taconic Trails Map put out by the NY/NJ Trail Conference? It's a big help for loops in this area because it shows some old fire roads that aren't actually named or marked trails, but are quite easy to follow if you know where they are. Anyway, I would suggest a some variation on one of the ideas mentioned:
Start at Undermountain Trailhead on 41. Go up to the AT, over Bear Mt., about 100 yards before you get to where the Paradise Lane trail intersects, where there's some boards over the trail, go left on an unmarked trail for about a mile. You will pass the AMC Northwest Cabin, and soon come out on Mt. Washington Road. Go right and immediate left onto an old trail/fire road. In a couple hundred yards, bear left on a red blazed trail. This goes over Round Mt., then over Mt. Frissel then past the CT Highpoint, then past the Tri-state marker. I think the tri-state marker is about 6 miles from your start point. If you're feeling energetic, you can keep going straight through the next intersection till you hit the South Taconic trail where you can go left to Brace Mt (very nice as mentioned). This is around 7 miles from your start.
Now, to get back, you pretty much have to go back up and over Frissell and Round, but it's not nearly as tough this way as going out. Now, when you get to Mt. Washingotn Road, you have two choices: go back the way you came (past the AMC cabin, etc.) but go left on the AT and a quick right onto Paradise Lane trail back to the Undermountain. Or, stay south on Mt. Washington road for another couple hundred yards and then go left on an old fire road that takes you to the AT 3/4 mile south of the summit of Bear and 1/4 mile North of the Undermountain trail back to your car.
This is a great trip with wonderful views from several summits. The area around Round and Frissell looks and feels a lot wilder than it really is. It is truly a pocket of wilderness with development all around but not very visible.
Another suggestion, start at Mt. Washington Forest Headquarters (a couple miles south of the closed road up the West side of Mt. Everett). Head up the Alander Mt. Trail to Alander Mt. Either take a left on the Alander Loop Trail, or just a bit further on the South Taconic Trail, merge into or stay on the South Taconic trail all the way to Brace Mt., then backtrack a bit and go right (East) on the Mt. Frissell trail and in 1/3 mile, go left (north) on the Ashley Hill Trail which goes all the way back to the Alander Mt. Trail around a mile from your start point. This is a real loop with minimal backtracking, probably closer to the high end of your 10-15 mile range, but with not nearly as much climbing as the previous trip.
Of course, the trip up Everett and Race is very nice, but hard to make much of a loop out of, and also hard to get over your 10 mile range, even if you go all the way across the ridge south of Race and back. Both peaks and the ridge are about a 9 mile hike total.
I just keep coming back to these mountains again and again, they're so accessible, very beautiful in all seasone and you can get some pretty good challenge out of them if you plan your routes right.
Good luck!
