Grizedale has some of the best riding in the Lakes but you need to know where to find the trails. I'll give you a clue, they aren't on the North Face trail.
Seriously though, a few minutes with a map and you should be able to spot the green lanes and bridleways that criss cross the whole area between Coniston and Windemere. The riding there is all relatively low level and you're not climbing for hours on end before you get a good DH run.
Having said that, you should also do at least one of the big passes – Nan Bield, Walna Scar, Gatesgarth etc.
Have a look at the Bogtrotters website. They do loads of riding in the Lakes and they have a phenomenal library of photographs for all their rides.