The phgysical conditioning thisng can't be over-emphasized. You'll be in an entirely different element and it will be completely unfamiliar to you. Then you're going to be in water that's cooler than you are. The typical sensations that come with exertion are either not there or masked. You'll be astonished at how sore & tired you'll be- and have little memory of exertion. For a guy over 40, this can be a real problem- alot of times you don't realize your in trouble until it's too late.