As above, you can't really go wrong with any loop out of Dentdale and back in again. It comes down to some degree to how steep you want it. All routes are distinctly up apart from the road(s) to Sedbergh, which undulate downwards. In order of severity: Barbondale (up from Gawthrop); up the valley through Cowgill to emerge at Newby Head; turn up the Coal Road at Cowgill; up into Kingsdale via Deepdale (bear right from Dent, then right again). It's all good but my favourites of those are Kingsdale, then the Coal Road, and then Barbondale.
Starting from Dent I think the best 60Km (just over) route would be Deepdale, Kingsdale, up Oddie's Lane from Ingleton and then past Ribblehead, over Newby Head and down to Appersett, then across to Garsdale Head and up the hard side of the Coal Road to emerge at Cowgill and roll back down to Dent. Not an easy ride as such, but extremely scenic
63km
The easiest loop would be down to Sedbergh, keep going left and you'll end up in Barbon and can go up Barbondale on the gently sloping side, which is very fine indeed though only a little over 30km.