Hi Russell,
I reckon the best way to measure up a track beforehand is to use Google Earth. If you haven't heard of it before, it's a program you download from the Internet and install on your computer. It gives you access to satellite photos of almost anywhere in the world. It also allows you to draw up a track and measure the length.
Here's the site you need to go to to download the program:
earth.google.com/download-earth.html
Once you have installed Google Earth and you've used it to find the area you're looking for, you can select the ruler at the top of the screen which displays a small menu bar on your screen. On that menu, you have the option to select "path". Once you do that, you simply click your mouse anywhere on the map to draw the track you want to paddle. The little menu box automatically measures the distance for you.
I've attached a screen shot of a random track I drew up just now. [Change of plans, seems I'm incapable of getting a photo up without making a mess of it. Let me know if you want some more details and I'll flick you an email].
hope this helps.