Several companies make small round patch types that work very well. Try them out on long runs before the marathon. It is very handy to take an old set of sweats to wear around before the race and then discard them. I don't mind looking for the sweat truck after a 5 or 10 K race, but after the marathon it's not fun. Any additional clothes taken to the start get left at the start.
Puddles Watch out for puddles along the way.
At the water stops and elsewhere water collects. If you step in a puddle you could spend a long time running in a wet shoe. It can cause lots of discomfort and blisters. While there are some portable bathrooms along the way it is handy to know that most businesses and homes will let you use their bathrooms. On Heartbreak Hill there are portable bathrooms every few feet but they are often in use by the spectators.