Just what I was thinking.  Bring a hydration pack and have a couple of extra bottles of water at every aid station.  If you need extra, grab them, otherwise leave them behind. 

20 miles is a long way, if it were me I'd at least take a full hydration pack (2L) , and that might not be enough, especially if it's a warm day. And I'd practice with it before the event. Can you have a drop bag? If so you could have a bottle waiting at the 36 and 100-mile aid stations for those long legs.