mitchy I have been doing Body Attack once a week which has been great, it is quite "interval" ish so think helped with my speed. Also try to do a gym weights session to build up strength (do not always succeed).
Fallen off the wagon a little recently due to injury but looking forward to starting again on Monday when kids are back at school.
I think a little bit of anything else alongside the miles is useful, and mixing up the running a bit too (different distances, pace, routes) is fun.
Obviously I do not take my own advice at all times :)