Welcome to the wonderful world of fell running!
I did the Three Shires as a first ever fell raca in Sept 2006 and Borrowdale the following
summer (!!)
I think a lot of it depends on your aims and perspectives - for me it's taking part/finishing and a sense of acheivement - I'm never going to be quick or threaten any records but it's all about getting out there.
HOWEVER, As far as Borrowdale and the majority of the Lakeland classics IMHO I think experience counts for a lot.
Yes - you can up your mileage, yes you can train on the hills and that indeed will get you round but there is nothing more satisfying than navigating your way off a mountain top in the the thick clag with a following of runners who couldn't be bothered to learn how to read a map or use a compass.
Fell running on these races is a far cry from following the crowd of a trail race say so if you can I'd certainly recommend at least trying to familarise yourself with the area whether that be getting up there at least once or twice proir or pouring yourself over a map and recognising/marking the route and checkpoints etc.
I'm sure there will be lots of useful advice to follow though!