Here's one that works well: Find a long, gradual hill and run four or five repeats (about 3 minutes each) uphill at a moderate, relaxed pace. Jog back down for a full recovery between efforts. Gradually intensify your hill sessions just as you would in interval or speed training. Add one or two repeats to your session every week until you reach 10 repetitions.
As you reach your racing season, reduce your volume of hill training to one relaxed hill session a week. Warning: Beware of steep hills. While they can build great lower-leg strength, they also can greatly stress the Achilles tendon and plantar fascia (the fibrous tissue in the soles of your feet). So remember to stretch these areas well after your workouts, and indulge in periodic massages.
Downhill running is a bit trickier and should be approached with caution, because it involves more impact on your body than uphill running does. Choose a fairly gradual hill with a soft, even surface such as grass or dirt. Run downhill at a fast but controlled pace for about 30 seconds, then jog easily back to the top. Gradually build up to eight to 12 repeats. You might include downhill training once every 10 days or so to improve leg turnover.