Joe
Do you want to walk just the wall , or are you interested in a "classic" walk from coast to coast . There is a very well known route , originally written up by Wainwright, from St Bees to Robin Hoods Bay . I think it is about 160 miles and generally takes about 8 to 10 days staying in B & B and Guest Houses en route.
There are several commercial companies who will arrange everything and carry your heavy luggage between stops, or you could do it self supported . Truly great country and well worth the effort.
One commercial company is
http://www.mickledore.co.uk/walking-holi...4FUgxoCqqzw_wcB But there are many others , I can't make a personal recommendation, but could make enquiries on your behalf if that is what you wish to do.
Reasonable fitness would be fine , but the weather can be wet , sometimes very wet, but not too cold unless you wish to do it in late autumn to early spring
Andy