Ooops, I signed up for an Ironman. Am I Doomed?

I think you're sorta answering your own question here. The Nice bike course has a reputation of being one of the tougher ones, so you can expect to be pretty well trashed by the start of your marathon. If it were me in your current condition, I wouldn't jump into IM. Get at the very least a year of experience doing shorter races. That may not be the answer you were hoping for but it helps to be realistic about things like Ironman. It really takes a lot of training to be well prepared.

Hang on now I'm a little confused. Are you talking about doing the Ironman this year? Have you already been training for 8 months? What has your training been like up until now?

[getting popcorn.....]

I think you are in for a world of hurt.  I won't say you can't do it because determination and will power can go a long way but it won't be pleasant.  I suggest spending as much time on your bike as possible between now and then being ready to walk the majority of the marathon.

I do wish you luck and hope you succeed.  

There is only so much adaptation you can force in 2 months.  Even if you had unlimited time to train and sleep between now and race day, you'd just be digging yourself into a fatigue hole, that your body only has so much time to recover out of. 

You are not, and will not be prepared to do an IM.

Can you finish it?  maybe.  As stated above, if you're willing to put up with a lot of discomfort (pain) and have the determination to keep on moving forward, then yeah, you might be able to finish.  Provided you're an otherwise healthy individual. Race day is the icing on the cake.  you haven't made the cake.

So, if it were me, I'd cut my losses now.  If you really want to do an IM, then give yourself another year and take your training seriously.  Get into it, have fun and see what you can become.  Just suffering through a day, isn't exactly proving to yourself what your made of.

but- if you are going to go through with it... might I suggest taking your race day nutrition very seriously.  It's the one thing you can still do right.

You can't do too much with the running, other than increasing the distance of your runs gradually otherwise you will risk injury.  At least you have covered the distance of the marathon although you can probably expect your time to be slower after the swim and bike in an IM.  At least it is pancake flat.

The bike course is tough, although I found it very enjoyable.  What type of bike are you using?  I am sure you are well aware of the long climbs, but the descents can play a big part in your overall time too.  I chose to put drop bars on my Tri bike with clip on aerobars and I passed many many people on the descents who were riding a full tri set-up.  Given there are almost no flat sections the slight loss in aerodynamics with clip-ons, in my opinion, was outweighed by the extra control and speed on the descents.

To finish within the 16h cut off I would say you need to be getting off the bike at the 10 hour mark, giving you 6 hours to do the run.  Subtract your swim time from this and that gives you an idea of how long you have to cover the 112 miles on the bike.  Then train like hell on the bike so you stand a chance of meeting this time (still fresh enough to run for 5/6 hours...). 

Best of luck.

There have been folks (including a few here on BT) who have just shown up and done an IM with little or no training.  So you can't say it can't be done.

That being said, you're in for a potentially long and unpleasant day. 

Two months to go means you really only have about six weeks to train hard before you would need to start tapering,  Not a lot of time to accomplish much. 

The challenge with making the cutoff for an IM isn't the paces you have to maintain, it's the lenght of time you're going to be out there, i.e., all day.

If you do the HIM, I'd suggest using it as a training day, practicing the paces you'll be doing in the IM.  If your try to 'race' the HIM, then you risk putting yourself into a position where your recovery impacts your IM training, which is your real goal.

Good luck, use good judgment and be safe.

Mark