What should be my marathon goal time?

I have questions about my up coming marathon next year. I am going to run Big Sur international marathon next year on 4/28/13. I have about 21 weeks to train for this marathon. But I don't know what should I aim (time) for this marathon. My time are below:

10k = 40:10

Half marathon = 1:41

Marathon = 3:59

I only did one marathon and I hitted the wall on mile 20. The last 6 miles took me about 1 hour and 15 mins for my last marathon. I run about 20 ~ 25 miles per week. I did three 20 miles training run last time. Should I go above that? How can I increase my speed? imporve my time? How do I avoid to hit the wall? If I run too much during one week my knees will hurt.........What should I do? What should be my goal time?

Thank you so much for your help.

You lack endurance. This is obvious by the massive drop in pace between your 10k and hm times. You need to run more miles, 20 - 25 a week just isn't enough to build a good endurance base. You should be looking at at least double that.

If your knees hurt, get them looked at by a physio. They really shouldn't be hurting unless something is wrong.

xtraining for endurance is best if you really can't handle the running. Anything - cycling; swimming; rowing machine; xtrainer. Build on your overall fitness.

I think that you set off way too fast for your last marathon. 3:59 is just over 9m/m yet your last 6 miles were run at 12m/m. You need to set off and maintain 9m/m pace for at least the first 20 miles, so run your long runs at 10m/m ans add a faster paced run at 9m/m. It's quite likely that you have the potential to run a marathon in a much better time but until you can or are willing to put in the miles it is unlikely for that to happen.

Try building up the mileage slowly with slow paced running, find out what is going on with your knees,maybe avoid to many road or hill miles to begin with, although you do really need to train on similar terrain to that of your marathon if this is going to cause you problems initially then maybe best avoided.

Really there is no alternative for running than running. Consider why you want to run marathons so much if you can't put in the training. Maybe stick to shorter distances or settle for slower times.

As Bex said, you really need to work on your endurance.  Have a look at the HADD training plans for ideas of how to boost your base fitness.  You really need to do lots of miles at low Heart Rate to significantly improve your marathon time.

I wouldn't worry too much about your goal time at the moment.  You need to increase your weekly mileage, keeping your Heart Rate at around 70% of max (or at a level where you can still hold a conversation without gasping for breath).  You can then tailor your training for a specific time in Feb.

+1 for stutyr's Hadd comment. I have a similar HM time and struggled to finish 6 marathons properly.  I adopted Hadd, trained by heart rate, and slow running meant I could boost my weekly mileage from 30 to 70 miles a week without injury.

I also put in 7 x 20+ milers (at 70% maxHR)

Result: Last marathon I beat my goal time with a -1 minute negative split. 

Note Big Sur has a huge hill in it - so do some hill work too!

Good Luck

Get the miles in and you could do a sub 3.15 based on your 10k pace