Hi Rachel. My best advice would be to just get out and go for a hillwalk. If you feel like running part of it then crack on! Don't be overawed, all but the very elite runners walk most of the steep inclines up anyway. I am a middle of the pack plodder and consider myself to be a glorified hill walker rather than a fell runner :-)
Being serious though, when out in the fells or hills let someone know where you are going and when you intend to be back. Take more clothing than you think you need - you can always take it off if you get too hot but if you are too cold and don't have an extra layer it can be miserable and potentially dangerous. Final advice - always, ALWAYS! take a map!
These are my opinions based on learning the hard way ;-)
Oh Granty your post is music to my ears! I thought I'd take a different route up and out of the valley this evening and ended ankles deep in mud. I laughed and huffed up the hill, there was no chance of running on the mud slide. It's heart warming to hear that not everybody has legs of iron and infinite lung power.
I love maps and still have my trusty compass; I shall do as you suggest and take myself off to the hills. Pen y Fan is near and a nice walk, maybe I'll just run down for my first trip out
Many thanks for your advice!