Not quite Tom, the benefits of training can last quite a while.....especially when they are maintained with other exercise such as football (which is a terrific way of keeping you in good running shape), you hint that you are slim too, so its not like you've apparently piled on the pounds in some couch potato state.
It would have been rather more transparent to say that you dabbled with athletics at high school in 6th form and you have recently run a marathon with what many would consider to be a low level of training.
For what its worth, in my first race since high school I posted a 39:38 10K off minimal 'running' training. But It'd look really disrespectful to all the other posters who comment in threads such as 'Can I do a sub 40 min 10K with this training schedule' if I were to neglect that I'd spent the previous 14 years training very hard in martial arts/boxing and gym work.
But no hard feelings mate.........at 22 I probably was guilty of exaggerating a touch or neglecting to tell the entire truth in all manner of situations (especially to the ladies, or to my mates.....about the ladies, or how much I could drink, bench press, how fast I drove my car......whatever!!). We mature.
What you can take away is that right now at 22 (if that is your real age
) you can run a solid 5K. If you want to get very good you could work your way into a decent weekly training plan. I know a local runner who took up running in his mid 20s who has recently ran a 15 min 5K.........and believe me, he wouldn't have been close to 18:09 at 22.
A great book to read would be the Daniels running formula by Jack Daniels (not a joke....thats really his name!). Its on Amazon.co.uk. Its quite technical but simple to follow.......seeing as you are of 6th form stock, I'm sure it'll give you every answer you are after and more.......best of luck.