For right now I've selected Dieting, Exercise, Nutrition, Supplements, Equipment and Mental Health as my main topics.
Using Joomla makes it very easy to change this if I want at a later date. Even if those main categories contain sub categories and articles, they can still be changed around.

I've spent a couple hours (I admit it, I was watching TV while doing it - after all its Sunday) doing some keyword research for these main topics.

I found both http://tools.seobook.com/keyword-tools/seobook/...