We like Sale Fell for a short walk (about hour and a half) but also love going round the lakes. Ennerdale is a good lab walk (about 8 miles on path and some rougher terrain and a small bit of a climb) with no sheep. Crummock is also a lovely walk but there are sheep in places so have to go back on the lead. Its about the same distance as Ennerdale.
Id avoid Loweswater as although it is beautiful dogs and humans are strictly not allowed in the water so its torture for labs.
Buttermere is a lovely walk and you have the option of styaing on lower ground for a 3.5 mile walk round the lake or being adventurous and heading up to haystacks for a bigger walk. Theres a lovely farm tea/gift shop when you get back to the car park and too and they have loads of tables outside where you can sit and have a cuppa, some lunch or a homemade ice cream
Hope this helps. We are in the West of the lakes so only really know our "local" walks but we have invested in some pathfinder lake district books and are finding them useful.
Heres a pic of Gem in Buttermere just a couple of weeks ago:-
