The most critical part might be before that where you have to veer off-path for 1.8 miles to Howden Edge after Back Tor. If you're doing a solo recce, starting from Cutthroat Bridge and working out a route back there might be more useful. I've heard that 20 miles should do it, which is precisely what I did last Saturday from Edale to Cutthroat Bridge and back (just plotted on Tracklogs).