One of the most overlooked challenges of a hilly course is the demand the downhill portion places on your muscles. Of course, you have to prepare for the uphill and most people seem to do this well. However, you also have to prepare to run downhill. The uphill, although difficult, is not what sets you off. Whenever you run there are concentric and eccentric contractions occurring in the leg muscles. Running downhill places significantly more forceful eccentric loading on your legs than running on flat land. After the repeated nature of these eccentric contractions, your legs will begin to lose contractile force, resulting in a slowdown and heavy legs. That is, unless you specifically prepare for this in your training. The key is to incorporate small amounts of downhill running (at race pace) into your weekly runs. Your body gradually begins to buffer this accumulation of eccentric warfare on your legs. How much and how often will vary greatly from runner to runner. In general, this should be progressive in nature.
Don’t ignore the demands of your next race. Training should be purposeful and specific to the course you plan on racing.