I know you have already run the race, but for next time: Wear a Garmin watch or similar, and go out at exactly the intended pace for the first mile. When I first got one back in the day, I was literally shocked at how slow it seemed for the first km or so - and especially for the first two minutes. I ran a PR.
After that eye-opener, I realized that by utilizing a grip on proper pacing, I felt like I was getting a mile (or at least the first km) almost for free - it felt like I was warming up as compared to running by feel, when I gave it 'race effort' from the get-go, not fully understanding that it really should feel easier than most people think in the first five minutes or so of the raced (if you are an 18-minute guy).
Another thing that really helped me was to not be afraid to fully warm up. I used to run a nervous mile or so, then I went from the low 18s to high 17s by doing the above, but also by beginning the practice of running the entire course slowly before the race. I'd do it in about 24-25 minutes, timing it so that I would end this warm-up with under 10 minutes to go before race time. Then I would take time to get to the start, get ready, and do some strides for the final minutes. At first I was apprehensive, thinking I was wasting energy, but it really worked for me. I was able to fully warm up my working muscles, and also focus on what needed to be done, alone with my thoughts. Not to mention having a complete understanding of the course I was about to race. YMMV.