How do I know what time is realistic?

Alison asks
Last year I ran my first marathon (3.35) and was happy to finish but somewhat disappointed with the time given my half marathon time was 92mins. I thought I would have easily gone under 3hrs 30 for the marathon.
When choosing a training time goal, how do I know what time is realistic? I would like to be able to run the next marathon in under 3hr 20, but not sure if this is realistic?? I currently run 5k in about 19.20. My last 10k was 40.40. I am 34yr old Female.
Lee's response

Hi Alison,

It seems that from your times that you have given that your half marathon should be closer to 90 mins and your marathon should be 3hr 10mins  - 3hr 15mins.
I am unsure of the training you did previously but it indicates to me that you are lacking miles in the legs (strength/ endurance).
I think your goal of 3hrs and 20mins is very realistic from your personal best times over 5km, 10km and 1/2 marathon.
A secret to your marathon time is to double your 1/2 marathon time and add 10 - 15mins.

Good luck and I hope you achieve your goal.