This is just my view/opinion:

1) After some quick math: You ran the first two kilometers in 7:25 and the last 2K in 7:32. If you split the third K in half and add it to those two times you get 9:18 for the first half and 9:25 for the second, for a total time of 18:43. (The extra five seconds may be due to rounding the splits or the time it you to cross the starting mat?) The first half of the race was 49.69% (9:18/18:43) of the total time, the second half 50.31% (9:25/18:43).

My experience (and from what I have read) shows that if the first half of a race takes 49% to 51% of the total time, you ran the race in the most optimal way. My opinion: if you ran the first kilometer 9 to 12 seconds slower, your total time would have been the same - give or take a few seconds.

2) Based on your splits, heart rates, description of how you felt at the end of the race, and the fact you had fresh legs; I think you ran to your current fitness level. You can run faster, but it would be due to things like a faster course or cooler weather.

Congratulations on meeting your sub-19 goal.

If you consider yourself a slight newbie, you did a great job of pacing for one. Yes, you opened a little fast and I'm sure it cost you a little. If you'd swapped the 5th and 1st Ks, I'm sure you could have found the extra gas for 3 sec.

Ideally, your 5th K might not have been 10sec faster than the others though....my best races I usually feel like I'm all out the last K, but in reality it is perception and I'm just a few seconds faster than the other Ks. That's when I know I left it all out there. However, it's really, really hard to plan it that close unless your on a 200m indoor track.