recently I reverted to some training strategies that I used while racing slalom.
While I am no longer striving to be the best slalom boater that I can possibly be,
I remember the overall benefits that training gave my creeking and freestyle boating.
A large part of it was using a heart rate monitor to train within certain ranges.
It's something that applies to every sport, and every level of fitness, with incredible benefits.
The main idea is to build a strong aerobic base, upon which you can build a larger capacity for work.
In whitewater, that means that you have the strenght to pull off the more difficult moves and the health to keep doing it.
There is in incredible amount of information on the internet and in books about his subject, so worth doing your own research. But, as a very quick introduction, consider these links.
Start with 220, then subtract your age, you'll get a good estimate of your max heart rate, which is how you then calculate your exercise zones
after you have copied your excercise zones down, you are ready to compare them to the chart that will help you figure out your exercise targets, which include ways to gain general fitness or peak conditioning.
if all that is too much, and you have never done any kind of focused training, try a couple of weeks training in the "moderate daily activity" and "weight management" zones. That will give you a healthy base upon which to build.
If you are an athlete, but have not paid attention to your HR, consider alternating days of Aerobic Training and the Anearobic Threshold training.
In addition to following doctors' advice, I have found that the best trick to making sure that I was not over working, and ready for more intense workouts, is by taking my heart rate every morning when I first woke. If it's faster than normal, then I need more rest and will modify my workout. If it's normal or getting a little bit slower, then I can keep working hard.
disclaimer: this is all based on my own experience. Everyone is different. Talk to a doctor before you go and workout.