Muscle pain and inflammation following a race, speedwork, downhill running or other hard workouts is known as muscle soreness.
At the end of a hard run, race or marathon, walk rather than stop completely. Then cool your legs off with cold water over the entire leg. Take anti-inflammatories, stretch often and, if possible, get a massage. After a marathon, take three days off from running and cycle instead. Pick up running again on the fourth day if you want.
To help prevent muscle soreness, add some downhill running to your training every two to three weeks, especially when you're preparing for a downhill race. Your quads will thank you.