how long between 26.2 and IM

How long it takes to recover depends on many factors...

How long have you been doing endurance sports?

How long have you been running?

Are you at your ideal weight?

How do you recover?

How old you are?

I do understand you concern about the run in IM. But I do think you would be better served to train for triathlons and skip the Mary. Do some google searches and try and read the article Endurance Nation talks about marathons during the off season. If you push to do a Mary I'd do it the end of the year prior to training for IM which would mean the fall of 2012.  3-4 months is not ideal for most of us, consider 3 week taper and 3 weeks recovery that is a 6 week hole in your training.

Building a base in running can't be rushed. Doing so will likely get you injured, burned out or both.

I can't answer this at all but, I was just talking about this to my wife because I had an marathon in my mind, but it is only 1 month prior to my IM.  I look forward to seeing answers to your question!

I see while I was posting a perfect response to my situation and will pass.

Cathy's questions are good to answer for yourself.  An experienced marathoner with a huge base and years of training is one thing but starting out new is quite different.

So, for the OP, I would try to put a little more space between the two events in 2013 as I explain below.

(For smithe68, good choice on passing it up.  One month would destroy your IM. This is because the marathon would be smack in the middle of your final IM build period and right before peak.  Not a good time to be training for, racing in, and recovering from a marathon IMHO.)

My own history and experience with this: I'm a long time runner.  I've done a marathon and an IM in the same year and have another marathon and IM this year. This year, the time between is 19 weeks, which puts me at the end of Base 2 for IM training and beginning of base 3. 

Both races are important to me this year so I'm OK with that.  If the IM was the priority, though, I would probably not do it and here's why.

My bike training during the 3 weeks before and two weeks after the marathon is limited.  Because of this, I would not run a marathon any later than 15 weeks before the IM (assuming the IM is important to you).

My swim training is also limited when training for the marathon because of time constraints.  I'm still improving my swim so this time off is somewhat a concern with me.  On the plus side, I get to focus on swimming while recovering from the marathon.

Last, a marathon takes me about 5 weeks recovering with a maximum of only 3 "recovery" runs a week before I can get back to running regularly again.  During this time, I am really prone to injuries if I even push it a little. This will also impact my IM training.

thanks for the comments.  I guess I don't really have a gauge of what it takes to "recover" I was really looking at the mary as a long training run in the training process.  as I mentioned, for me the stumbling block is mental and over the run--I figured make sure I can get past that hurdle then move to the whole shootin match and was looking for that magic time where I can sustain what I know I can do.

FWIW, I'm relatively new to endurance sports--always intrigued, but way too lazy.  Was a mediorcre but active athelete in HS (wrestling and football), lifeguarding, wathersports and LOTS of whitewater in college.  Came to this last year when my dad had tripple bypass and I realized I needed to make a change.  I've done 3 sprint tri's to date with middle of the pack performance  (had planned an OLY last season, but got injured anddialed back).  I tend to be top 1/3 on swim, middle of the pack on bike and bottom 1/3 on run.

nowhere near my target weight, but WAY better than I was when I started and expecting to drop at least another 30 lbs or so--but don't expect to ever not be a clyde.

I also find that I dont' really have a training speed versus a race speed--not sure if that matters.  It's more like an on/off switch.  My bike improved at touch last season and I'm hoping to pick up my run pace as I progress, but after a year of 3-5 days a week of running I'm still at about 11:30mm--good days closer to 11 bad days closer to 12. I can push a 10 on the treadmill, but can't sustain that pace by myself--fastest run to date was a 5k at 10.5mm pace.  that pace is gook for a 2 miler or a 5 miler. 

thanks for your suggestions.  I'll look further out in 13. 

falconsprint, a marathon is great to do for itself but you will find that there is very little in common between a stand alone marathon and the run in an IM. 

A few HIM races before your IM would be a better confidence builder for the IM that you eventually race.

I'll give you the short, simple answer.

it takes a month to recover from a Marathon. 

Nevertheless, to do a marathon requires a few weeks of taper, running the 26.2, and then a few weeks of recovery... before you can get back to your IM training.  Putting a marathon inside your IM training might make you emotionally better prepared- having run one, but will leave you physically less prepared (believe it or not).  Going through the marathon process leaves you less fit, than having just steadily trained through that same time period.  Plus, the likelihood of injury is higher in preparing for a marathon than an IM.

does that make any sense?

these are two admirable goals.  I'd pick one at a time.  When I started, I trained up for a 1/2 marathon... and got injured and missed it.  Then I trained up for a full marathon... got injured and missed it.  And I started out in pretty good shape