I was in this same type of situation and I would win 5ks by 30-60s, and I once won a 10 miler by a mile, literally. I would hang around afterward, eat the cheap bagels and drink the sports drinks and chat it up with the other local runners, but never let on that it was less that full effort. It was always, "yeah, I had a pretty good one today", but usually I directed the conversation at them.. what was your time, was that your best, I can't believe this weather.
I used to do my cool downs with some slower guys and add on afterward. I became well respected and not regarded as cocky or standoffish. I never wanted to be the guy who showed up at local races only to have people look at me like, "Great, that asshole again." Instead, I got to meet a lot of nice people who had the same passion as I did, just at a slightly slower speed.
It just takes realizing that in the grand scheme of things, we all suck at running in comparison to the best of the best, so let's enjoy our passion for the sport instead.
Before I knew it, a rep from Avia shoes had offered me some free gear/shoes as a mini sponsorship. I didn't pay for shoes for a couple years. Not bad for a 15:45-16:00 guy.