Because of where I live, if I run outside, I run hills. I really don't consider it a special part of training. Just get a few routes going that include hills for everyday/easy runs first. After you adjust to that, then we can talk hill workouts. When it comes to leg strength, lunges (i can't spell and that does not look right) will help a bit. Did you injure your hip flexors at one point? Did you ever try looking up PT excersizes for hip flexors since you know it is a problem for you? Hills will not be good if the problem is just a weak point in the body. And if all you ever ran was not far from the 30 mpw that you are running now, it might be best just to run more if you plan on running marathons. That alone could take care of a good part of the problem.