I love Wiltshire, it's so full of places to explore. Trails are the way to go. I run and bike on isolated tracks all the time, even in the dark. I am never scared. I do carry a mobile always and tell husband where I go. I don't have any road shoes anymore as never use them now but Asics do a few good trail ones, I got a good pair from Sports Direct for £30.00 recently and love them. Cannot comment on the knee situation apart from go and see a good physio, chartered one if possible, I could recommend one but I'm in Oxfordshire so a bit way off. Anyway, good luck with trail running, it's great fun especially in late autumn when the trails are muddy, it brings out your inner child to come back home covered in mud.