Be careful there. You sound like me, and I have a story for you.
I'm 30 now, PR of 4:36 for a full mile, 5Ks in mid 17's, just running for enjoyment but not training to race. My road 5K PR is 16:36, my half marathon PR is 1:18, and my marathon PR is 2:53.
I had a friend in college who ran 4:15-ish in high school, and did not run for his first 3 years of college, not a step, during which time he porked up from a racing weight of probably 145-150 to a healthy beer pong weight of about 185.
Then, junior year of college, he decided he wanted to run a marathon.
It is probably relevant to note at this point that his motivation to run a marathon was his twin brother having just run a 2:28 in his debut effort (age 21, training solo, not on the college team). Their mother's PR is in the 2:55 range. Athletic family.
After his 3 years of zero running followed by about 2 months of jogging, at which time he was probaby still about 180 pounds, give or take a couple of pork chops, we raced a 5K together.
I had been running regularly and was in good shape generally, but made no effort to peak or anything. I ran the 5K in about 17:15 or so.
He kicked my @ss.
My not trying to peak was irrelevant. He obviously wasn't trying to peak either, and I've only run faster than he did in that race 3 times in my life. After 3 years of beer pong and 2 months of jogging, he would have kicked my @ss pretty much no matter how well I ran. I'm glad I didn't have any money riding on that.
Epilogue: The next month (i.e., 3 years of beer pong followed by 3 months of jogging), probably still a pork chop or two over 170 pounds, he ran 2:50 in his poorly-paced marathon debut, beating my half marathon PR en route.