Here, copied from my old post at RWOL, are a set of questions, the answers to which will help experienced marathoners give less-experienced marathoners pace advice.  Please use these as a template to start your own topic, rather than commenting here (which I'm going to try to turn off).  If you're an advice giver and would like to add a question, PM me, or start a thread with your suggestion and all of us can discuss it.

1) What are your recent shorter-distance race times?  This is the most useful indicator of your marathon pace; there are lots of calculators that will take in a race time and spit out projected times at other distances.  Some well known calculators are at http://www.mcmillanrunning.com...unningcalculator.htm and http://runworks.com/calculator.html, and gmaclin (who posts here) has a nice spreadsheet calculator downloadable from his website http://mymarathonpace.com that lets you set how aggressive you want the prediction. Half and 10K times are best for projection, but if you have multiple distances, including 5K, that's useful as well because they will indicate how your pace falls off with increasing distance - a measure of endurance.
If you aren't racing shorter distances before your marathon - well, you should be.  But if you're not, a solo time trial can suffice.

"Recent" means within the past, oh, 2-3 months, tops.  Six months is not recent.  Two years is definitely not recent. (However, if you ran a half marathon six months ago, and a marathon five months ago, these are useful, because their relationship will suggest how your more recent half will relate to your proposed marathon.) "Race" means that if you did not run all out, if you ran it as a "training run" (and, seriously, why would you do that?) it is not a race.  Please give your race times in total time and distance, not in pace per mile or pace per km.

2) What's your average mileage per week? More miles tend to translate to more endurance and less falling-off of speed with distance.  If two people have an identical half marathon time, but one runs 40mpw and the other 60mpw, the second is likely to do better at the marathon.  This means over the 10-20 weeks prior to your race, not your single peak week.

2.1) What training program (if any) are you using? Are you running a midweek "medium long" run?  How many long runs (18-20) have you done?  Raw miles are the most important, but how they are distributed is important, too.  Higdon's weekend loading and Pfitzinger's medium long runs are good tools for building endurance. 

3) When is your race, and which one is it?  If you're running next weekend, our answers are going to be different than if you're running next month.  If you're running CIM, our answers are going to be different than if you're running Big Sur.  Please note that "about to run a marathon" means "going to run one in the next few weeks".  If it's 4-8 weeks off, you have time to run a tune-up race or time trial of at least 5 miles first, which will help refine your pace.  If it's more than 2 months off, any pace advice is irrelevant. 

4) How long have you been running?  Speed gains come quickly for new runners, but endurance is built over time.  Drop of speed with distance is usually less in those who have been running for many years.  

4.1) How many marathons have you run, and what were your times?  Experience with the distance is useful.  How your prior marathon(s) matched up with your other distances is a good indicator. If you have sets of e.g. half marathon/marathon times, it's super-helpful.

5) What is your age and gender?  Older runners and women tend to lose less speed with distance.

6) What pace do you run your long runs?  For some reason, most people who are posting here for the first time seem to think this is important, and that the faster they run their long runs, the better their marathon will be.  This is exactly backwards.  Long run speed is a negative indicator; if you run your long runs much faster than the training paces implied by your shorter races, you're not building endurance properly and are less likely to hold your speed through the marathon distance.  

7) What is your goal?  If you're BQ or bust, maybe you'd rather go for a stretch 3:15 than a more-likely 3:20.  Conversely, if you'd rather BQ than PR, you might want to go for a safer 3:15 instead of a stretch 3:10. If you blew up and had a miserable deathmarch in your first marathon, you might be conservative this time to avoid a repetition.  

Finally, please remember:  Even with all this information, nobody really knows.

Good luck!  And remember, post your answers and pace info request in a NEW THREAD! 

Remember, by "about to run a marathon" I mean, running a marathon in the next 6 weeks.  It's not useful to have more than a general goal earlier in the training cycle.  If you are just in the early stage of your training cycle, and you have a general goal, I encourage you to join the appropriate goal-time thread in this forum.  Others who have similar goals (and who may have hit them recently) will have good advice for you!