Manta
It's a good endurance activity. As our Hawaian friend suggests try to combine periods of high intensity (beyond your anaerobic threshold, which is before you start to find hard to keep your breathing, and usually after 15-20 min of exercising) with moderate to low intensity (before the anaerobic threshold kicks in).
When you exercise past that point you'll use the glycogen stored in your liver and muscles to produce ATP (energy). Before that point you'll mainly use fat as energy source.
How to do know when you are here or there? Use a heart rate monitor, otherwise listen to your body and slow down
And don't forget to follow a sound nutrition program.
Ah! you can send me a cheque for $25 for this basic advice. My address is:duh
Sorry, it was for free
Gerard.