Let's see, does beer drinking (high school/college) or smoking pot (college) count as athletic activity. Other than the occasional bike ride, those were the things that occupied my free time during my "formative" years. No organized sport other than freshman football.
The first inkling that I had some aerobic talent came in Army basic training when I ran a 6:00 mile in combat boots and fatigues. I was the 3rd fastest guy out of about 160.
I did not take up multisports until my late 30's. Since then (18 years) I have done fairly well almost always placing in the top ten or on the podium at 1/2 IM or shorter. I am still an IM apprentice so my results are not as good (30 out of 120).
The way I see it, I have some advantage in that my legs are "newer" without that early competition. However, I had a lot of catching up to do on training, eating, equipment, proper form, etc. I was injured a lot (primarily running injuries) the first 4 years in the sport as I gathered knowledge.
My friends from the "old" days (beer/pot) cannot believe the transformation. Only a few of them compete or train regularly, while the others, sad to say, are on the downward spiral caused by a sedentary lifestyle.
My advice, read all the materials you can find, listen to the wisdom of the "old timers" in the sport, listen to your body to avoid injury and train with people faster than you for extra motivation.
Good luck!
"You know you are getting old when you see your father in the mirror." Anon.
Dean Wilson
http://www.anaerobiczone.com Bicycle Protection Indoors & Out