Start training in July 
Seriously, it's hard to offer advice without knowing what you've really done.... If you really must do this, then I'll give what advice I can... Although I've done quite a bit of running over the years, until recently, I've always stopped at 10 miles... so this advice is based on that experience... and quite a lot of reading round the subject of distance of running - preparing for my first marathon this coming Sunday.
1/ if you've never been to a proper running shop to buy shoes, then go - this afternoon if possible. Take your current running shoes. If they're friendly, and not busy, they might well advise on a range of things. They might say you don't need new ones. But it's best to check - you are about to put your feet and legs under a whole load of unfamiliar stresses, and you need footwear that protects you. It is very possible to buy shoes that will actually harm you if you take on all this running.
2/ Whilst you're in the shop, even if you don't buy shoes, you can 'pay' for their advice by buying some twin-skin blister resistant running socks. Also ask about something to protect yourself from runner's nipple. Something I'd forgotten about until recently! But most runners will suffer excruciating pain from time to time as your shirt rubs your nipples with every step.
3/ look at the half-marathon training plans on the internet. make an honest assesment of where you might be (it's pretty clear you will be a long long way short of what you 'should' have done... but if you are as fit as you say, you should at least be able to get round the first half marathon.
4/ In the next 3 weeks, get some SLOW runs in. Everything should be slow. As you're a gym/pool/sportsperson, you'll probably need a pace that makes you feel a bit embarrassed at how slow you're going. Don't do anything that involves speed running (I'll presume your other sports/gym will have given you a modicum of sharpness). Save ALL your running for building endurance. Don't try to do TOO much.... there's simply not time, and you're likely to pick up a nagging injury. If you're as fit as you say.. and you do some sports that involve running (eg footy) then it wouldn't surprise me if you could launch into 6 miles straight away. Then just run this sort of disance every other day... with a 9 miler on the Sunday 2 weeks before.
In the penultimate week, I'd say Monday rest, Tuesday 7 miles Wed 3 miles easy, Thursday 3 miles easy, Friday 6 miles easy, Saturday rest (or swim) , and Sunday 10 miler easy . Then hardly do ANYTHING in the last week. Literally do a 20 minute light jog on perhaps the Tuesday and Thursday.
All the mileages I just mentioned are very much finger in the air... and are over-ambitious if you are greener than I've given you credit you for. Be honest.... and if you really should start at lower mileages then do...
This is not at all ideal. Normally, you would build up milage slowly over a period of 14 weeks or so... then start to 'taper off' your training for the last 2-3 weeks, not 6 days.... letting your muscles get fully repaired and fueled up. So, I'm just trying to condense all that. But be prepared to stop if you get injured. What I'm proposing is a bit of a risk... and it's up to you to decide if this cause is worth you risking injuries.
Before EVERY run... spend 10 minutes loosening up (ok... in practice 5 minutes might have to suffice... but if you do this, then run the first mile even slower!). To warm up, start by walking, then jogging and flexing all the muscles you'll use. Don't "stretch" before your run. After EVERY run... warm down by slowing to a jog, then walk
