I think cardio is very important but so is some type of load bearing exercise. If you can combine the two, all the better. I find a bicycle spin class benefits me more than any other. It's an incredible cardio workout plus every muscle in your legs screems and the rest of you body is working harder than you might expect. I try to do at least one organized class per week and then ride on my own two more times each week. I find that the organized class makes me push my limits more and sets the standard for my solo rides. By crossing your anaerobic threshold, you expand the window where you can stay aerobic.

I also like lap swimming and of course walking or hiking with a pack is a must.