I am signed up for a half marathon in January.  I signed up for it months ago, as running a half marathon was one of my New Years' resolutions.  As I have trained for it though, I have increased my mileage to the point where I have decided on signing up for a full marathon.  The closest one I can make it to is on December 19, while the half marathon I am already signed up for is January 8.  That's only about three weeks inbetween––is that going to be long enough for me to recover from the marathon?  
Any long-distance runners here that have some experience with this?

Marathon = 26.2 mi

You should be fine. Took me 1 1/2 weeks to recover from my half marathon

Uh, yeah.  I meant how long as in the time between...

That's a tough question to answer without knowing:

-your weekly mileage/training volume going into the marathon
-how hard you intend to run the marathon
-how hard you intend to run the half marathon

Some very simple, generic guidance is to do no hard running for 1 day/mile raced so for a marathon no intensity for ~4 weeks.  If you're daytripping the marathon or running the half as a training run vice a race, you'll probably be OK.

Well, my training volume so far is about 25-30 miles a week spread over 3 runs.  My longest run so far is last week, which was a 20 mile run.  I worked up to it gradually, going from 14 to 16 to 18 and then to 20.  I do the really long training runs every other week, with a 10-12 miler as my long run on the off weeks.  Did a 10 today, for exmple, after doing a 20 last week.  
As for how hard I intend to run the marathon...well, I am going to try to average somewhere under 9 minute miles.  I was at around 8:45 on my 20 mile run, so that's my goal for the marathon.
Not terribly fast, I know, but it's my first marathon, so I don't want to push it.
I think I can do under 8.5 minute miles for the half marathon.  How far under, I don't know.  I can do 8 minute mile average on a 10 mile run, but it drops to around 8.5 when I take the mileage up to 12-14, usually.  Being that this is a race, though, and I tend to run faster in a race than in training, I might be able to take it to 8 minute miles for the whole 13.

you should be fine.  I know several people that run the Disney half marathon and run the full marathon the next day.

I was gonna be a wise ass and say 13.1 miles

4-6 hours or you will get cramps....

Uhhh...if I ran a half marathon 4-6 hours after a full marathon, I would get more than cramps, I would get just about dead.

Hence my question about goals.  Those folks who do both either:  1) don't run the half anywhere close to their best effort or 2) have large bases.

Rik - with a typical weekly mileage of 30, if you run the marathon at or near your best effort for the distance, you'd be better off taking it easy (no hard/intense running) for at least 3 weeks.  You can still run the half, but don't expect a PR performance from the race.

To help speed your recovery from the marathon stay active in the days following the race.  Don't go right out to run in the first few days, but go for an easy swim the next day, maybe the one after that.  Then go for an easy ride or two.  You can start doing easy/short runs after 3-5 days if you feel up to it.  Don't just prop your feet up for a week.  If you stay active and do a bit of running you won't give up much, if any fitness after the marathon.

This first reply is awesome by the way, and kudos to you for not going back and editing it.