Tough decision...need advice?

Getting an assessment from the doctor is a great idea to know for sure what's going on.  On the optimistic side, hopefully some PT or ART (if you have someone in your area) could make your injury happy again fairly quickly and you'd still be able to do your race.  Best wishes to you!

If you have to stay off it for a week or even two, it isn't a huge deal, providing you can come back and do some decent running in mid to late August.  Assuming you are currently on track, you won't lose much fitness in 1-2 weeks, and you can add bike training and even pool running.  (Pool running is about as exciting as mowing the lawn but it helps a lot to maintain fitness when you are injured.) 

Laying off longer than two weeks, this close to race day (and presumably you've got some long runs scheduled during this period), and I'd be worried, unless you have a huge reserve of run experience.  If sub-10 is a the whole point of the race for you (rather than just "It'd be nice..."), I'd reconsider doing the race.

I've done an IM without proper run fitness going in.  It sucks.

Ah this is your first IM.  I'd definitely go, then, unless Doc says you'll just hurt yourself.

Time goal is great and I totally encourage that sort of ambition, but anything can happen in the face of the unknown that is your first IM.  Enjoy the day, pay attention, and even if you don't get the time goal, you will have learned a lot.

Try some deep water aqua-jogging while you take your break from the running.  I've usually found that helpful when dealing with running-induced "itis" conditions of the leg.  Good luck!

I agree with ^^ this ^^^.

Water running can maintain a lot of run fitness.  The one thing to watch out for is that while it will maintain a lot of aerobic fitness, and even motion-specific endurance for running (if you do it well), you will be losing resistance to the pounding and the severe eccentric loading that your legs take during running.  It can actually be a two-edged sword because your legs might feel pretty great in your first runs back on the road (that's one edge, the good one), so that you don't realize that you are exceeding the ability of your muscles, tendons, ligaments, etc., to deal with the pounding (that's the other edge, the one that cuts).