I am loathe to give specific training tips, but just a few general pointers.
If you are a non-smoker with reasonable body weight who cross trains you are half way there.
I personally do not use gyms but your local YMCA gym could be of help to you.
In my opinion I believe nothing beats time in the boat—I suggest a minimum of two reasonably long paddles a week in the period leading up to the event, with an aim to paddle 4–5 times a week in the last 4 weeks leading up to the Marathon.
Provided your health is reasonable, age is not really a factor. I am in my late 60’s and still seem to be able to get down the river and enjoy the trip. Our oldest competitor was the legendary Ted Jackson who completed the 2003 event with me in a double sea kayak when he was 82 years of age.
A final tip—Laminate copies of the maps for the 5 days and stick the map of the day on the deck of your boat. The distances between checkpoints are marked, so by using your watch and the average speed for your type of boat, you can have a very good idea of where you are and how long it is to the next checkpoint. I use a GPS which gives me my speed and distance covered which is very useful.