If your friend is in good shape, you *could* try River Road, which runs parallel to 9W. Plenty of space, very few cars, but plenty of hills, especially that last one leading back up to the police station and back out onto 9W. Then again, you can just skip the hill when you get to it and turn back around. Also, the Hudson Path north of 180 is nice and secluded. Or go past the Bronx and up Saw Mill Pkwy bike path. That one's nice!