hi avit
its been awhile since i posted anything on here and just read your post, i got into fell running by fell walking, fell walking/hill walking uses the same muscles as running up the hills does, i'm based in whitehaven cumbria, i take it doncaster will have some hills ? try cycling, kettlercise , hill walking to build your stamina and leg strength, swimming is also excellent for some time out from the pounding of running but still gives you a fantastic workout.
if you are near hills try running up and down them, build up slowly to prevent injury. you will soon notice your fitness getting better, hill running always builds up stamina anyway
hope this helps, let me know how you get on, also if you ever come up to cumbria and want a run up here let me know, also check out the running groups in your area, i love running by myself but i also find group running keeps up the moral especially if the runs and hills are tough
GOOD LUCK !!!
CHARLOTTE