Even though the hills may not be long in terms of getting up them, they are steep. There is one hill I routinely run on one of my routes that, I kid you not, is at least 30 percent grade or more at it's steepest. You have to lift your leg quite a bit to make it up that. You consistently run these steep hills year after year and that adds up.
Something else I have done since I was a kid that probably has helped me. Yaqui made reference to this in his post: Running stairs. Not so much in training, but in everyday activities involving stairs. I probably go up and down the stairs in my house at least 25 times a day. I sometimes like to see how many stairs I could take in one step (I believe my record is 6!) Even simply going up, I will usually take 2 or 3 stairs each step. Often I do this sub-conciously. Maybe all of the years of this type of activity has also contributed to developing my hip flexors.
Something else I have done since I was a kid that probably has helped me. Yaqui made reference to this in his post: Running stairs. Not so much in training, but in everyday activities involving stairs. I probably go up and down the stairs in my house at least 25 times a day. I sometimes like to see how many stairs I could take in one step (I believe my record is 6!) Even simply going up, I will usually take 2 or 3 stairs each step. Often I do this sub-conciously. Maybe all of the years of this type of activity has also contributed to developing my hip flexors.