True story.
Met a homeless gentleman. He was not strung out on drugs or anything and was in solid shape for his age. Just a bit beat up from life. He was at a park sitting on a bench.
Another guy had on a T-shirt from a road race and the homeless man read it out load and then mentioned that he had been a runner. He asked a couple of questions about the race that an average person wouldn't likely ask. Then in return the guy asked the homeless man when he ran. He said the years and then named his PR's. These were PR's that only a runner would know and they lined up for 800m, 1500m and 5k. He wasn't bragging, just answering the question. We looked him up. The guy had been an all-American and competed at the US Champs / Olympic Trials back in the day.
You never know who you will meet and when. Someone who was a good runner years ago isn't like a pro basketball or football player where their size would be an indicator of their history. Tons of former great runners out there of all ages just living life.
I don't know your co-worker obviously, but a 3:45 1500m from 35-40 years ago in the 1980's wouldn't be outrageous. Very do-able. That's about a 4:02-4:03 mile. Lots of good college guys did that. Now if he said 3:35 in the 1980's then that's another level.