It`s awesome - because you should not be comparing yourself to other people - but where you were a week , a month or a year ago
There are a lot of factors that effect your own pace - hills + humidty are 2 biggies
The average pace if you wish will be effected by sex and age aswell as environmental factors.
you`ll probabrly run a little faster in your race - becuase it`s a race - and you`ll put a little spurt on at the end and push yourself in the middle - so figure an 11 mm - unless this is some special elite race you managed to signed up for there will LOTS of people behind you - and more than likely one of them will be a guy half your age.- But I don`t know the race - so maybe you will be last - if you are so what ?? .. you will have completed your first 5k - you have won the second you put your running shoe on the starting line.
Good Luck - enjoy yourself !!