..to all these jokers telling you NOT to do it. DO IT! What reason is there to run a marathon? Ask everyone who does one. Making money is a good reason, but for everyone else, whatever your own personal reasons are is enough. That's enough for all the MB naysayers. They run marathons too. All this talk about Kenya being so far ahead, etc., etc. then when someone actually wants to get with the program instead of waiting until they're 30 (and a few years older by the time they've gotten used to the training and learned the event, missing their prime years) there is uproar in the streets. Who says your actual goal is 5k? The MB trolls, that's who. Not you. YOU want to run a marathon and should.
These same posters who scoff won't be on the NYC starting line trying to stop 20,000 people who 'have no reason' from running. They're just hassling you be cause you're on the Board talking about it. You don't need a reason that is meaningful to them. As long as it is to you.
I did one (under 3 hours) my first year of HS. 5km was not my main event - I only ran one ever. My training consisted of 70 miles a week or a little more. You have a better idea than I did what to do. Run up to 23 total. Do a fair amount of race-pace training up to 15-17 at a time. Ignore those who don't approve because they would never do it. ESPECIALLY ignore those who feel they have the right to but you don't.