Running hills vs. flat terrain

Running up hills all the time can hurt your achilles tendons.  Believe me, I know from experience.  It does also work slightly different muscle groups than running on flat. 

If you're specifically training for a flat race, I would train mostly flat (or vice-versa).  For general training, I try to do a mix of both. 

A lot of people underestimate the amount of stress that downhill running has.  You're quads are working a lot harder going down hill because you have gravity + weight.  That's why Boston is such a killer course.  I've heard that if you aren't ready for the down hill, it becomes a VERY long race. 

I've been doing squats and lunges getting ready for it.  Also, we have a "hilly" 5 mile loop in a park that I try and pound out to get my legs use to it.  The first couple of times, my knees and quads were killing me.  Just be smart about it and your body will adjust.  I personally think that if you train on hills, the flats are much easier.

I run a hilly course about once/week, maybe once every other week it just depends on other factors (time of day increases traffic, mostly).   Later in the season I'll replace the moderate hilly run with hill repeats, maybe.  I don't plan to do any hilly races this year.

 If you are going to specifically train on hills I would say do it no more than once/week, and make it your "short run".  My typical hill repeat run isn't more than 35 min, and it takes about 7 minutes running time to get to the hill.  Also if  you are in your first season I would absolutely leave it out for this season, as it's very stressful on your joints/tendons and will increase your risk of injury.

I feel that hill training improved my running.  Especially since I really really hate speedwork and never do it.