I have done this loop - at least most of it - as part of the Tour of NYC century ride. It's not great, but it's do-able on a bike with slightly more substantial tires, such as 700x25-28, or certainly a mountain bike. Lots of small rocks, uneven pavement, etc... We did not use the trail exclusively however, and spent some time on the parallel roads as well. It is pretty, though, and it's nice to see that part of the city on a bicycle.