In reply to jimjimjim:
I haven't. But anyway my top tip is to get someone who knows to look at your forward paddling and optimise it as much as you can. Make sure you are using your big muscles in your back and core. If you paddle with your arms you will get tired a lot more quickly. Try to find your natural "gears" and somewhere in there is a speed and rhythm that suits you, not too slow, but one that you can sustain. Get as many miles under your bum as you can. Take climbing tape for your fingers.
Have fun.