Following up on SRD's idea...
I've not done much running - I just don't seem built for it, despite at school being lanky and people always saying 'ooh, you've got a long distance runner's build'. Hobbled round a couple of half marathons in hours and hours.
Two years ago I decided to do a 10k - of course choose a brute going round Arthur's Seat road twice. Did a bit of training, got myself to a slow 1h8m.
Fast forward to last week, take it upon myself to map a 5k route from the house, went out at 11pm, just over half an hour, reasonably pleased, calves ached for days afterwards. But I'm doing this for fitness for the cross racing, so went out again last night, same route, a couple of minutes under half an hour for the 5k point (the route is a smidge longer), calves not so bad today, and even managed a couple of sprints on the bike.
Let's see how long this lasts. I'd quite like to do a sub hour 10k, but I'm not sure I could run further, it's just boring in comparison to the bike - the reason I go out late at night is I just enjoy the quiet streets, always liked that kind of thing, and seeing foxes scurrying about ahead.