Marathon training run & sprint tri

I've been doing this juggling act myself recently.  This is my first season of Tri's which I'm going to end with a 1/2 iron event which is my "A" race for the season.  However I have a couple of sprint tri's in the middle of all of this as training races.  I don't want to compromise my 1/2 iron training at all, but at the same time I wanted to have a good race in the sprints and try and guage myself against the rest of the pack and my age group.  I've found that as long as I have a complete rest day the day before the sprint that I can train pretty hard right up until that point without any trouble.  My first sprint tri was a couple of weeks ago.  On the thursday prior I did a 56 mile bike / 3 mile run brick, and then a 13 mile run the next day on the Friday.  I took Saturday off, and then raced the sprint on Sunday.  I placed 22/214 which just barely made the top ten percent of the field.  So I was pretty happy with that.  I think really it all comes down to how quickly your body recovers.  I think the more you do to speed that process (stretching, ice, compression, massage, etc...etc...) the stronger you will be on raceday.

So if I were you I would consider doing the 12 miler on Friday morning (giving yourself the max amount of recovery time).  If you're really worried about it affecting your race then bump the 12 miler to Monday and do it at a more relaxed recovery pace.  I would not recommend skipping it. 

Have a great race and enjoy your marathon training!