I've been trying to take my race board out in both lite and high winds, and experimenting with the foot strap placements to find the fastest set up possible. Having your weight to the stern of the hull seems to be the fastest, but it also takes the most amount of effort to keep the board trimmed right..
The legs and the core muscles like your abs and lower back seem to take the most strain, but really everything needs to be strong and tight to keep it on course, especially in heavy conditions.
Something that builds strength, stamina and balance with out a lot of impact would be best.
I've found that mountain biking works well for me.
I just got a GPS, and I'm going to try and use it to fine tune my form even more on the water.
Good luck dude!