Three critical things: Credit card. Sunscreen. Food & Water.

300 miles in familiar territory? Pack light, think of weather, and rather than be overly concerned about mechanical failure, inspect and lube the bike up before hand, and have a list of LBS phone numbers within hitch hiking distance of your tour. I probably wouldn't take any more tools than my day ride under-seat bag already contains, but might throw in an 8/10mm wrench for my fender stays. Focus on packing light and having fun.