Too Much Strength
Many are surrounded by hills, such that every run has regular ups and downs.
Others always run on flat terrain but hit the gym for power lifting to try to 'stay strong'.
Some runners love hiking and do so regularly on Saturday with their long run on Sunday.
Though each of these scenarios can have some benefit to building strength, stability, and durability, if any are taken to the extreme, one key component of your running mechanics might suffer - ground reaction time (i.e. a quick response to the ground after making initial contact).
If all you do is run on hills, or hike long the day before your long runs, or do only slow, powerful strength training, it is very possible that you are losing and neglecting your 'quick feet' - a vital component for peak performance.
To keep your training balanced and progressive, you may need to drive to a flat area a couple days per week, or hike only after your key runs (hiking on tired legs is better than running slow on 'tired from hiking' legs), or incorporate light and fast plyometrics into your strength routine.
Unless your goal is to simply endure a hilly course, balance your power on the hills with fresh legs on flat terrain so you are ready to hit your pace come race day.