What do you do to stay fit?

I am in my low 50's and find that I am doing well to stay out for 3 hours - but I will really feel it later. If I surf more than a 3-4 times in a week by back feels crappy and my legs will give out on me after 1.5 hours in my next session.

I am particularly interested because in July I am off to the Maldives for two weeks and I am wondering how I will survive two sessions per day!

Any tips?