ok, i can't be arsed to read all the replies nor argue with some of them.... and screw the gym, save your money.
its really simple... swimming, swimming and swimming.
but not just a doggy paddle nor 2 legnths as fast as you can then into the jacuzzi, get in the sessions when the pool is laned and swim to a plan with front crawl.
like anything build an endurance base first. if you are new to swimming, first off time yourself over 400m. that will show you how shit you are - i was well over 10mins 18months ago when i tried (6m55 when last timed, could probably go faster now.. we'll see, being timed again in two weeks).
then go with 200m blocks x 6 or 8 and build up to 400m blocks but all the time work on technique, google swim smooth and the like, swimming is all technique.
when you can do 3 or 4 x 400m comfortably start taking the middle block as intervals at pace. 2 x 50 - 2 x 100 - 2 x 200 etc
the trick with swimming is to enjoy it, its almost zen like getting into the groove of improving your stroke technique and breathing sequence. medititive as well because you are head in the water looking down.
i really wish i'd latched onto swimming years ago, its the biggest single improvement i've ever done to my surfing. i can't surf anywhere near as much as i would like, often weeks out the water, and although my timing and technique may suffer from these gaps i can get on a board and paddle long and hard with no trouble at all and, when you get the usual dicks who try to out paddle you, i can run circles on em. actually find it quite hysterical now when people try it because i just rip their arms off.
beyond swimming anything that boosts aerobic base is good, cycling, running etc.
stand up paddleboarding, flat water, stunning summer flat day work out. head out for a 5k med pace run, you'll know you've had a workout after.. just get a free windsurfer off freecycle and a cheap used paddle, tape up the dagger board slot, and get out there. 50quid tops.
also, something i'm just beginning to understand after 46 years years, stretching and working on core muscles. you don't need no gym kit...http://www.physioadvisor.com.au/8110250 ... y-exer.htm
finally diet... watch the booze, avoid white carbs and don't eat high fat.
so endeth the sermon!