That's a hard question without knowing a lot more about you -- like saying "how tall should I be?" --- but I'd say that for a female who's never run before, 8:45 - 9 minute mile is too aggressive. I know a lot of women who have run for years and still run at about a 10 minute/mile pace, or even slower. I also know many who are a lot faster. Everyone is different; there are a thousand factors involved and there's no "typical" or "ideal" pace.
For your first race, I'd say any finish time is a good finish time. Just enjoy the environment and the experience! A new runner in their first race shouldn't be concerned about their time. Otherwise you're likely to start out too fast (a common beginner situation), then get really winded and have to walk (also not uncommon, and walking is not a bad thing), and because you pushed too hard you overdo it trying to reach that artificial time you set for yourself. Just forget about that for now. You can't really set a goal finish time until you have run for awhile and have learned what a good, comfortable pace is for YOU.
Over your 10-week training period, focus on gradual, slow, steady improvement, not speed. Trust me on this; there's plenty of time to work on improving your time in future training and races.
Most 5K's I've raced have had "average" finishing times around 30 min, which is around 9:20/mile. That does vary from race to race, of course - depends on how flat or hilly the course is, weather conditions, etc etc. I'd recommend looking up past results from your race on the Web for a more exact number. (It may or may not be on active.com; try the race website).
As for your projected 5K finishing time, a good goal time could be perhaps 40 min...or it could be perhaps 20 min. There are a large number of variables: What is your level of natural talent at this distance? Have you been doing other aerobic exercise? How is your general physical condition? How are your running biomechanics?....etc. You are young, so age is definitely on your side. If you have not been running before, you will be better able to project a goal race time after you've been training for about 2 months (that would put you in mid-December).
@ 5K: Ontario Mills 5K, Ontario, CA, 25:17
New Balance Palm Springs 5K, Palm Springs, CA, 24:32
Angels Baseball Foundation 5K, Anaheim, CA, 24:24
@ 10K: LA Chinatown Firecracker 10K, Los Angeles, CA, 52:15
Great Race of Agoura - Old Agoura 10K, Agoura Hills, CA, 51:40
Fiesta Days Run, La Canada, CA, 49:57
Dfit... I think that you hit the nail on the head with your response. There is no way to give a good answer for a starter. Some are in better shape and can finish sooner, while others take longer.
For where I am at, times for the 5K for this past weekend (cold weather, rain) was 19min + for the top guy. The last person crossed at 53 minutes. There were 2 ladies that had run in the past, in their early 30's. They just had babies and the one lady had not run for year. Their time was around 32min.
As for your starting time - start out training (cannot remember, are you using the C25K plan?) Use your initial time as a base and work from there. Improve upon that and keep going.... DO NOT over do yourself... I have been there and am still dealing with shin splints for bad shoes and poorly planned training program.
Since this is your first 5K race ... use it as a bar to improve in the future. Really, as a newbie, you are running against yourself. Keep practicing and enjoy what you are doing. I never would have thought that I would have looked forward running in the cold, etc....
My race times (2012)
Race Schedule for the year:
May 6th Frederick Running Festival 1/2 Marathon
May 28th Run through the Park, Ligonier 5k
August Tentative - 5 miler in Chambersburg, PA
October Baltimore Running Festival Marathon (also the 2nd part of the MD Double challenge)
Personal Best :
5K : 28:32 Run through the Park, Ligonier PA
5mile : TBA- Marine Corp 5 Miler in August, Chambersburg PA
10mile : 1:44:52 Cherry Blossum Festival Run, Washington DC
Marathon : 5:01:22 Akron Marathon
10/10/09 Lake Joe Pool Pumpkin Run 5K - 33:15
11/26/09 Dallas Turkey Trot 5K - 30:35
02/14/10 Austin Half Marathon - 2:12:25
03/13/10 Dash Down Greenville 5K - 29:27