You've heard about Heartbreak Hill at mile 20, but the elevation drop early on in the race can be just as tough. When you run downhill, your muscles undergo eccentric contractions as the muscle lengthens. The steeper (and longer) the downhill, the more trashed your quads will feel. The first 15 miles of Boston are either downhill or rolling and that can really fatigue the legs. To prepare, incorporate downhills into your long runs and speed work. Body weight exercises like lunges and squats can also strengthen your lower body and help you deal with the demands of the course.