Boston is a downhill course!
...that a series of eccentric strength exercises done two weeks before a downhill run reduced both DOMS (delayed onset muscle soreness) and the amount of strength loss. The researchers believe that the prior training does two things to help your muscles adapt to the stress of downhill running. It builds more sarcomeres which build a longer, stronger muscle. The prior training also teaches your neuromuscular system to dole out the force more equally among your muscle fibers reducing the chances of damage to one motor unit. -runningtheplanet.com
It's not too late to make a huge difference- if your legs are not prepared for the downhills I guarantee you will be very sore after this one. According to this article just one or two hard eccentric workouts will help.