Park run problems

I did a park run for the first time yesterday.
As someone who regularly runs five to eight miles - five easily - I saw it as an opportunity to work on my pb (personal best).
Needless to say, my PB was the last thing on my mind.
Park Run UK is a weekly event staged in towns and cities all over the UK where runners of all abilities can join together and run, walk or jog 5k around local beauty spots.
I know what my body can do and how I can push it. I also know it takes me about 15 minutes to settle into a run and get into my stride. So I don't know why I was surprised with my speed and that it felt tough. By the time I had warmed up I was already half way round and had the mentality of just wanting to finish.
I did it in 27 minutes 58 seconds. Slower than the previous 5k race I did which was around 25 minutes.
I came 94th out of 300+ so it could have been worse for my first go while I got to grips with the mechanics of the race.

A detailed results table

Battling through the start line dodging all the slower runners was a nightmare for me. My patience is thin which is one of the reasons I love running alone - another obstacle I faced during the run.
Ducking and diving around other runners also proved difficult, I felt I had to over take a lot which uses extra energy that I like to save on just going forward. 
Next time I will try to start nearer the front.
The results table updates online automatically every week helping you work out your PB. I know I can do better and would have loved to but now it gives me something to work towards.
Next week I have been roped into volunteering so I will have to wait until the week after but that gives me two weeks to train. And train I will. I want to get under 25 minutes.
I am realistic I know that it is unlikely I will go from 28 minutes to 25 straight away so if I get under 27 I will be happy.
I do love having a goal.