Run in college, any college. I missed out on college track because the small NAIA school I went to had XC but no track. You miss a lot of 5k/10k development by missing college track. You can attend college then run a marathon after XC or track season. You've already run a half-marathon in 1:12 so I don't see a problem with running a marathon at age 18 when you've already done a half. Alberto Salazar did his first at 18.
Saying you only have so many marathons in you is just plain bunk. That's like saying you only have so many 5ks in you. Pure bs. Now, you might only have so many good years of training in you but that's a different point. What usually happens is that runners will wait until they are in their mid 20s to run a marathon then by the time they put in 5 or 6 'thons they are in their late 20s and going downhill. So that's where we get this "You only have 5 or 6 marathons in you" BS.
Bill Rodgers ran 2:09:55 in his his 6th marathon and 2;11:08 in his 37th 7 years later. Average that puppy out, 4-5 marathons a year.
Alan