1/2 IM to Marathon - training suggestions?

If it were me, sure.   I'd pick up at 15 miles or so.  I'm no expert, but I think this is one of those situations where you really need to listen to your body.  At the first sign of injury, I'd shut it down and forget about the marathon.

Assuming you were recovered by the 19th, I'd do 15-16 miles.  18 on the 26th, 20 on the 2nd, 20-22 on the 9, 12-15 on the 16th, 26.2 on the 23rd.  From most plans that I've seen, there is a rest week in the middle of that, but your schedule doesn't have the time.  You could always cut out one of the 20+ runs too.

Are you just trying to finish the marathon, or are you planning to go at it aggressively?  I'd only do this if you are just trying to complete it.  

For myself, I'd say that I had an 80% chance of doing this without hurting myself.  I have no idea what kind of runner you are though. 

This advice is only worth the paper its written on ....   

That's what I'm doing.  After my HIM, I thought "hey, I'm at the same mileage as my friends training for the marathon."  I took it easy for about a week and then hopped onto their training plan.  I've had some other medical issues come up that has derailed my training a bit, but I haven't had any injuries from doing HIM-to-marathon training.  I'm not a runner by nature, but I'm also just hoping to finish the marathon (not be competitve at all).

My biggest advice would be to allow yourself enough recovery after the half ironman.  Especially since it sounds like you'll be giving a max effort at the race, you may need more downtime than you think.

Good luck!!!