I am doing ironman arzona in November, Is it crazy for me to do marine corps 3 weeks prior? I've never done an ironman distance before, so I am not sure what my taper will be?
Yes, that is crazy. Don't ruin your Ironman bid.
Unless you are already part of the cadre of people who regularly runs marathons once a month, this would be a very bad idea. Even without the swimming and biking, doing 2 marathons three weeks apart would be brutal on your body.
You wouldn't be tapering for IMAZ in the weeks in between. Instead, you'd be recovering from MCM. Decreased mileage and decreased intensity do not by themselves mean a taper. Marathons beat up your body, and typically take weeks to fully recover back up to previous fitness/health. Do that AFTER the ironman.
Do-able? probably. Advisable? absolutely not.
You could sign up for it, then run whatever portion of it that your training program dictates. Ex. if you have 3 week taper, this could be your last long run - run that length in the marathon and stop. I did that last year and it's a much more fun 18 miles than a solo run. If you have a 4 week taper though, this could be in your taper (depends on your dates), so you'd be doing say 10 miles. You could be better served looking into the Back Yard Burn 10 mile series that's going on in Oct-Nov. Caution though to run it at your typical long run pace and not push to marathon race pace. As Tamara said, never run a full marathon for IM training, unless you are one of those crazy folks who do it regularly. Remember, you are training for an Ironman, not a marathon. Good luck, IMAZ is an amazing time.
totaly agree with simon. Is better to run with a grou p than alone. I am doing Arizona too did you find a hotel ? When you are leaving need a training parthner let me know