As much as you might use your legs to push on peddles or grip the boat they do not go through any where near the range of motion of your paddling arms or your legs as they walk. Nor do they take anywhere near the amount of stress of walking, let along running or backpacking. But still many people find that the limiting factor of their time in a kayak is discomfort in their legs. This discomfort Symptoms Basically, any kind of achyness in the legs. More sevier would be numbness and tingling anywhere in the legs or feet.
Low back is more likely to be generally achy to more specific sharp pain.
Some of the low back problems may be related to your legs and some may not. Many people are short in their ham string and calf. Shortness develops as a result of daily activities. If you spend a lot of your day sitting in chairs or other postures with bent legs this will allow the back of your leg to shorten. When you then sit in a kayak with your legs straight you are asking the tissue to stretch more than it is accostomed to and it will complain.