Long distance riders have special demands for carrying nutrition and gear. For longer rides, a couple of gels and your pump in the jersey pockets won't always cut it.
You will wind up carrying extra food, additional energy drink mix, a patch kit and/or extra tube(s) or if you ride on tubulars, a complete spare tire. Minimal tools are also necessary, usually a multitool will do the trick. Depending on the weather, you may want to bring some extra sunscreen or chamois cream. You may also need room to store items of clothing that you take off after a while, such as arm warmers, or only need intermittently, e.g. rain jacket.
There are two different places to carry things: on your bike or on your body.
On the bike:
- handlebar bag
- rear rack (trunk/panniers)
On the body:
- in jersey pockets and under the hem of your shorts
- lumbar pack
Handlebar bag: usually won't work with aero bars, which a lot of ultracyclists use. I used to have a small one that came with the bike until I added clip-on aero bars. Advantages: everything is right there in front of you for easy access while riding. This is a really nice feature, and one that I miss. Some models come with a clear plastic flap that you can put a map or cue sheet in. Does not negatively affect aerodynamics. Disadvantages: the one I used had really stiff zippers and was difficult to open while riding without sending me across the road. Depending on the mounting system and size of the bag, it may conflict with your aero bars, bike computer, shift cables, or lights. Additionally, you also run the risk of being called a Randonneur instead of an Ultracyclist (1).
Rear rack: added weight to the bike, most carbon fiber frames don't have rack attachment points and p-clips can cause damage and premature wear to the frame. Might be considered overkill. There are rear racks available that attach directly to the seatpost. There are weight limits for these but they can be a good option if you only want a rear rack on part of the time.
As far as carrying on your body goes, if you don't mind the weight on your body you can also get quite a bit stuffed into your jersey pockets and in the thigh of your shorts. Gel packets are a good option for this location. You can also stuff a bottle or two down the back of your jersey, depending on how snug your jersey fits it should stay in place.
Daypack: plenty of carrying capacity. Can include a water bladder for in-flight refueling (e.g. the CamelBak line). Pretty much precludes easy access to jersey pockets, but you could still store soft flat things in your jersey pockets; disposable gel packets and other soft goods would be a good choice.
Lumbar pack: rides lower than a backpack. Usually doesn't have a water bladder, but a few do. Usually include space for extra water bottles. Also precludes easy access to jersey pockets.
I use a day-pack for commuting, and a lumbar pack for rides. The day-pack I have is a Gregory Z30. There are so many different options available for day packs that you really just need to go to a store (I recommend REI). Some things to look for: sternum strap, breathable mesh back, small pockets on the hip belt are nice for storing cell phone/mp3 player or lip balm, and side pockets for water bottle or coffee thermos. Make sure to try the pack on and weight the pack with what you think you your average load will be. I selected the Z30 due to the stand-off mesh back which lifts the pack entirely off your back, similar to an Aeron chair.
The lumbar pack I have is a Mountainsmith Tour with the Strapette accessory. The advantage of this setup is that there is sufficient carrying capacity, and the pack rides low on the back allowing ventilation. I adjust the hip belt so that it's snug, and leave the chest straps a little loose. The shoulder straps are pretty much a requirement, they will keep the pack centered on the small of your back instead of sliding off to one side. On the few rides so far with this setup, I really didn't notice the weight and the straps did not interfere with breathing.
As with backpacks, the idea is to cinch the hip belt so that it takes the majority of the load. The shoulder straps are mostly for stabilization. Although this is less of an issue while bike riding due to posture, you do not want significant weight hanging on your shoulders. You also really want a sternum strap. It doesn't need to be snug, it just needs to take up the slack so your shoulder straps won't part and slide off.
On the Tour I put two Platypus bottles (bags? baggles?) in the side water bottle holsters. I am a big fan of the Platypus products, usually even if I don't think I'll need it I'll go ahead and throw a filled bottle in the lumbar pack. Personally, I only put water in the Platypus; I'm a bit concerned about the ease of cleaning and wouldn't want something sitting in there and fermenting/achieving sentience. I have the several 1L and a few .75L bottles, the 1L is probably more useful because you store a bit more than a large water bottle, if you're really thirsty you fill your water bottle and then chug the remainder. If you put the bottle in an outside pocket, make sure you secure it to your pack with a carbiner or similar. When the bottle is empty, just roll it up and store it.
(1) This is an attempt at humor. I am not a professional humorist, nor do I portray one on television. Since my day job is not in the humor industry, there is the distinct possibility that my attempt failed. Please accept my most humble apology for besmirching the reputation of all humorists, satirists, jokers, jesters, comedians, and Randonneurs everywhere.