Just returned from a trip that started in Potomac MD at Great Falls ending in Harper's Ferry WV. 45 miles each way along the C&O Canal towpath. For us road bikers, 45 miles during an outing is a decent ride but certainly not a killer. Was much harder on the mountain bike.
First piece of brilliant advice I received from my bike shop is get a rack and a trunk bag. Don't attempt to carry everything on your back. This was a lifesaver as I did bring a tiny backpack for sandwiches and it just gets to be uncomfortable on a long trip. So I bought this rack
http://www.topeak.com/products/Racks/MTXBeamRackE-Type and this trunk bag
http://www.topeak.com/products/Bags/MTXTrunkBagDXP
Planned route knowing where access to water, port a-johns, and stores with food were along the path. And know what the store hours are too. Hotel was bike friendly and had a place to store and lock bikes.
Our plan was to spend one night in Harper's Ferry and ride back. We did. Was tough. But here's what I brought (and it all fit in the trunk bag): 2 spare tubes, pump, CO2
cartridges, tools, trash bags to protect stuff inside bag and one for outside in case of rain, chamois butr (will really need it), BenGay (lifesaver for neck
& shoulders and knee after I crashed), ibuprofen, Gu, carb drink tablets you can mix in water in your bottle,Handiwipe singles, Off bug repellent single wipe packs, mini sunscreen, protein bars, phone charger, bike lock, change of clothes & extra bike clothes for return trip. For the girls: sample packs of cleanser, moisturizer, foundation, shampoo & conditioner. mini deodorant, toothbrush,mini hair brush, ponytail holder, toothpaste, contactlens solution, case, mini eye shadow, blush, eye pencil, mascara, and baseball cap.
Bento box stored phone, camera and Gu. Kleenex in my jersey.