At the start of my 2nd season i was sitting on the beach trying to decide if I should head out as the conditions were 'iffy' (and I was being a wuss).
I had been watching a dude doing simple but smoove trstitions and turns in front for a while. He comes in to land, takes off his helmet and was at leat 60!
"Hell - if he can handle it I better kn get out there!" I thought.
Hey runstan, a very basic level of fitness and ability to swim a couple hundred meters is all you need. Err.. plus lessons of course. After the initial 'new muscles' soreness, you only need to go as hard as you want to really.
Get amongst it and (like I did at mid 30's) you will actaully feel younger and more agile soon enough.
Enjoy.