If you can find a 400m track, great. Otherwise, any big green space will suffice. Sprint the straights and walk or jog the corners for 10-15 minutes (depending on your fitness level). By sprint, I mean 90-100 per cent effort. Recover for 6-8 minutes.
No let-up, I'm afraid. Now run 400m and walk for 200m. Repeat this four times. Dig deep to keep your 400m times reasonably consistent, though expect some fall-off if you're pushing yourself. Recover for 6-8 minutes.
I know - there's not much left in your pins. So let's finish up. Sprint 100m; walk 50m. Sprint 200m; walk 100m. Run 400m; walk 200m. Then go back down in corresponding increments. Nice job. Now go home and rest. You've earned it.