Hills. You either love 'em or hate 'em. Most hate 'em. I tend to be in the minority and actually look forward to the hills in a run. Granted a flat course is great for a PR, but my body actually tends to fatigue quicker on a flat course. I think I need those hills to activate different muscle groups while giving the "flat-road-running muscles" a break. Sometimes a well positioned hill is just what I need to pull out of a running funk. Of course, you need to be conditioned to those hills, so when they appear they'll be a help and not a hindrance. I think it's many runners' avoidance of hills that make them dislike them so when the come upon one. Their bodies aren't ready for the physical demands of running a hill.
There are a variety of hill workouts that are great to sprinkle into your training mix. Actually, even if you're not in training for a race, adding some hills to your regular routine can really do wonders to your endurance and pace. It can also jack up your metabolism and help you avoid that extra weight gain that often comes with constant low-intensity mileage. Running hills involves all the leg muscles--calves, glutes, hamstrings and quads. When running the hill, you repeatedly fire all these muscles with basically no rest until the walk back down the hill. This builds speed and muscular endurance. It also involves activating some muscle memory so during a race when you come upon that sudden hill, your muscles won't be shocked that you're asking them to run up the hill at full force.
Google "hill workout" and you'll get a zillion different versions. I have three that I like to use with my runners--short hill repeats, intermediate hill repeats and long hill staggered intervals.
Short Hill Repeats: Find a hill that has a 5-10% grade that's about 100m. (Not sure how long that is? It's a hill that takes about 30-45secs to run.) Before the workout, do a 1/2-mile to 1-mile easy warm-up run to get your heart rate up and warm-up your muscles. (Never hop out of the car and charge a hill. That's a hammie pull just waiting to happen.) After the warm-up, position yourself at the bottom of the hill. Run to the top as fast as you can. Think 5K-race pace. You'll be running in or close to an anaerobic state instead of an aerobic state, so you're lungs may feel like they're burning and your breathing will be pretty rapid. Try not to let your breathing rate get out of control. Even though you'll "feel the burn" your breathing should consist of good deep inhales and exhales. Once at the top of the hill, turn and walk back down. This is your recovery period. Once at the bottom of the hill, turn and charge back up. A 4 x 100 workout would mean you're doing four 100m uphill repeats (doesn't count the walks back down). I recommend starting with the 4 x 100 and work your way up to a 10 x 100.
Intermediate Hill Repeats: This workout is basically the same as the short hill workout but it's done on a longer hill. Not a steeper hill, but one that has about a 6% grade and will take you about 90 secs to run (about 200m). Begin with the warm-up described in the short hill workout. After the warm-up, position yourself at the bottom of the hill. Your pace will be fast, but not quite as fast as the short hill repeat workout. Think 10K-race pace. Once at the top of the hill, turn and walk back down. This is your recovery period. Once at the bottom of the hill, turn and charge back up. Don't forget to use a good strong arm swing as you run up the hill. Arms bent at 90 degrees, swinging beside not in front of the body. A 4 x 200 workout would mean you're doing four 200m uphill repeats (doesn't count the walks back down). I recommend starting with the 4 x 200 and work your way up to a 10 x 200.
Long Hill Staggered Intervals: This is a tough but awesome workout. Find a long steady hill that's about a 1/2-mile long. The grade can be about 5-7%. Before the workout, do the warm-up described in the short hill workout. After the warm-up, position yourself at the bottom of the hill. Your pace will be fast, but not quite as fast at the short hill repeat workout. Think 10K-race pace. Run up the hill for 30 seconds, then turn and walk back down the hill for 30 seconds. Because you're walking, you won't make it back down to where you started. After the 30-sec recovery-walk down the hill, turn and run fast back up the hill for 30 seconds. Repeat this staggered interval process until you reach the top of the hill. For the first workout, one trip to the top of the hill will be fine. Eventually work up to 2 or 3 total hill climbs.
One hill workout a week is plenty, especially if you're doing other quality workouts such as a tempo run or long run. Hill workouts are pretty low impact, especially as compared to downhill running. However, people still often get injured doing hill repeats. Usually this comes from having weak hamstrings or glutes. It's a great idea to do some leg conditioning exercises to build some leg strength before adding hill workouts to your plan. Squats, hamstring leg lifts, lateral lunges, and step-ups are great exercises for building posterior leg strength.