The full Saco route, including Boom Road (so pretty) is 51 miles, but I've plotted a nice alternative: there's a cutoff onto Flag Pond Road at mile 18 that will shorten the route to 41 miles, if you're so inclined. Load them both, and happy riding (and maybe load the original route, in case you change your minds)!
Full Route:
Flag Pond Cutoff: