Surely one good tool for deciding what a good time is, is the Age Grading system.. There are several tools out there, but why not look at the Runner's World one.. here
http://www.runnersworld.com/cda/agegradingcalculator/0,7977,s6-238-277-415-0,00.html
This allows you to input your gender and race distance/times, and it gives that performance a percentage rating... 100%= World Class... 70% Regional Class 60% = Local Class.... with other scores inbetween.
For example, Janet, post #61... with a 10K time of 00:58:04 for a 56yr old women. That scores 64.6%, which is excellent for someone who's only been running for a short time, and achieved all that weight loss. I think the time would already mark you her out as a pretty decent club runner.
So to answer the question from the OP - I say that you can input your achieved times for 3K, 5K and 10K - and maybe also some 'target' times, and play around with it, to get a sense of average/good times.