In reply to MarkM:
If you get, or look, at this:
http://www.mtbbritain.co.uk/lake_district_mountain_biking_book_review.html
You'll see that most climbing areas have good mountain bike routes.
If you want to climb in Borrowdale then the long route round the back of Skiddaw is a good day out. In the same area is the "Borrowdale Bash" which has a fair bit of road work but the offroad stuff is good.
A really good longish day in Langdale is the Iron keld,Hodge Close, Loughrigg route. If you are camping in Langdale you can start/finish the route from Chapel Style.
The book has loads of other shorter and longer options and is well worth the cash.