Looks as though the suggestions posted thus far pretty much have you covered so this is just my $0.02 and may not be worth that much.
Unless you are just getting started, 55 miles should not take you all day. That said, carrying a meal (PB sandwiches) shouldn't be a big concern for a 4-hour ride. Food adds weight, but a better option might be individually packaged energy bars of some kind. It's likely that a light healthy snack an hour before the ride will provide sufficient energy for a ride of 55 miles. I also started filling one of my water bottles with gator-aid recently to start drinking about half way through a longer ride (30 miles or more). I have some routes with no water refill stops so I carry my camel-back and two bottles of water/gator-aid on those. One or two paper blue shop towels.
I agree with the idea of carrying a tube (or two)
and a patch kit. Around here the roads have enough debris on the shoulders that a ride of that length can easily result in more than one flat. A first-aid kit might also be useful in the event of a tipover and need for a bandage or pain meds. good luck!
now; let me see, I need to write down some of the other suggestions 