Just spotted this one , and have kayaking expierience .
First off decide what you want to do , canoe or kayak , there's a big difference .
No you don't need a licence , and they are no restictions as long as you are not trespassing .
However it is not something you just pick up , I would sugest joining a local club and learning the correct techniques , they will also have organized paddles so you will become used to moving water .
It's amazing fun , but it won't get you fit as you tend to go with the flow and the river does the work ( unless you get into playing in white water ) , it will increase your upper body strength though
Just give it a go , most clubs will have boats and kit they will loan out to potential members , so you've no excuse , go and try it .