Many paddlers suffer with chronic tennis elbow. There are some things you can do to relieve some of the pain. First be sure your paddling technique is correct. Don't bend your elbow to pull the paddle through the water; just let torso rotation do most of the work. A bit of stretching and warm up often helps you get off to a good start. An elastic band wrapped around the forearm, just below the elbows relieves some stress on the tendon and a thicker paddle shaft can help too. Wear a pair of padded bicycle gloves to give the thicker grip effect. If you don't have tennis elbow, good technique will probably prevent it from starting.
Back pain is the plague of sitting upright all day in a canoe seat; at least for many of us. A good way to relieve this is to outfit your canoe for alternately sitting and kneeling. Glue some three quarter inch thick mini cell foam to the inside bottom of your canoe right where you will kneel. Then outfit your canoe with a seat that will tilt down and back up so you can change position easily and often. The kneeling position lets you straighten out and relieves stress. The doctors and chiropractors tell us that sitting puts great strain on your lower back. A good seat back can work wonders too. Even though it is not good to use one in challenging conditions because it makes you more top heavy in your canoe, it is a great way to get some support and take a load off for a while. Good ones will last forever and make good stadium and camp seats too.