Unfortunately I am very experienced with back problems. A little inflammation/swelling of a disc will put pressure on the nerves, resulitng in pain or numbness. Be happy it is the numbness and not the pain. Sitting is the WORST position for this problem.
I have spent 1/2 of my adult lifetime, post surgery, in pursuit of a body that does not ache 24/7.
To have a back in shape, you need a stomach that is in shape. Stretching the legs is something to do, strengthening the abdominal muscles is a must to having a good back. The lower back muscles must also be stretched and strengthened.
I have been everywhere over the years from gyms to martial arts to yoga, to hanging upside to trying anything that even remotely might help a troubled back. The single most beneficial exercise I have done and do 5 to 7 times a week is sliding seat rowing. I also do some light weight training all around, nothing crazy, just steadily and routinely.
A person CAN spend too much time paddling a kayak. Mix it up. Ask your body to do different things, work all of the muscles in different ways.
I highly recommend supplementing kayaking and light resistance training with sliding seat rowing. The rowing beats jogging and everything else because you use 85% of your muscle mass, getting more for your time in fitness and calorie burning. Jogging would aggravate the situation with every step. Honestly though, be ready for a real workout, 15 minutes of rowing at 30 strokes per minute is not something you will likely be able to do daily the first week or so or three, depending. On the other side...gains will come fast if you put in your time.
I am lucky enough to have a rowing shell and a sliding seat rowing machine in my basement (Bodycraft VR100). The machine is for after work and I get out on the water with the boat at least once a week, many times twice. When the weather cools again, I will alternate time on the water between my shearwater and the rowing shell.
It takes a little time to row properly and call upon the muscles needed at the right time, there is a right way to do it. Start off easy until you perfect your form and gain some ability in endurance. Sliding seat rowing will help in many ways.
With pain or numbness in a leg, this indicates inflammation. Waiting for it to fully subside will not work. Slowly get into shape, slower than you normally would want to, giving your back a chance to heal, realign and support itself through muscle tone and flexibility.
The Oxford Shell would be a great compliment to paddling the kayak. In the meantime, a usable sliding seat rowing machine is a click away or just down the road at your neighborhood gym.
I hope this post helps someone out.
http://shearwaterkayak.blogspot.com/