The Five Dales route is nice, it rides up Back Lane and not down it I seem to remember, which is a pity, as that is a super smashing downhill, and a bugger of a climb. I haven't got the book here, but if I remember it right, you would be quite fit to do it in 2-3 hours.
This is a nice ride, 100% legal, and takes in a fair few of the best bits off the Five Dales route, but ends in a fabulous descent. I think I could ride round it in under 3 hours, it's only 15 miles. The first hill is a killer, but there is nothing else too crazy on it. Some great descents and lovely wooded singletrack. It starts from Darley Dale.
http://www.bikehike.co.uk/mapview.php?id=20299
There is a naughty weekdays only shortcut through Beeley Plantation that skips going down to Beeley, or you can spin down the obvious road shortcut to Limetree Wood and miss out Beeley that way if you want a shorter ride.
Joe