What's your longest run to date, 10 miles? Are you in any kind of PT or just recovering?
The reality is that if you stop now, you can probably make the distance based on your longest run to date, provided it's 10 miles or more, but you may not be running the entire way. And, by racing and training at all you are probably not doing your hip flexor strain any favors. The path to recovery will probably be to not run for at least 2-3 weeks, and to do some exercises that strengthen your glutes, core, and surrounding muscles, then reintroduce running.
I had basically the same injury, symptoms indicated I may have torn my labrum but it turned out to all be muscular. I was off running for 4 weeks (though I was cycling on my well-fitted bike - my not so well fitted bike made things worse - and swimming) and did 6 weeks of PT. I put it off until after a big race, which I successfully completed, but the more steps you run the more you reinforce bad muscle memory to compensate for your injury and the more recovery you'll have to do to reinforce new muscle memory.
Obviously I chose to race anyway, though that probably wouldn't have been what any sane PT would have told me was the best course of action.While running wasn't what "caused" my injury (I am pretty confident cycling with the wrong fit/pedals/saddle/etc did), it was certainly affected by it and probably exacerbated it, even in subtle ways that I didn't realise. My running tempo was perfect and symmetrical (which frustrated my PT
), but my hip movement was way off, and running definitely uses ye olde hips.
So... that brings us back to your question, which is a tough one. Like I said, I think you can make the distance if you choose to race and not do the 12 miles, though without that 12 mile run you won't be as prepared or as able to run your steady pace the whole way as you would if you had done it. I don't think you can stop running entirely for 2 weeks before your race if you're going to run it, either, aside from the fact that you'll go nuts you do need some of those miles to be best prepared. Wow, I'm torn on this one... honestly, I completely ignored it and did all the miles, then paid for it in extra PT time after the race. My race, however, was an Ironman, which meant a lot more miles and a lot more abuse than a half marathon, though to some degree it's all relative.
Food for thought I guess (I like to think out loud)... I'm not a PT or anything so I hope others will chime in.



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While running wasn't what "caused" my injury (I am pretty confident cycling with the wrong fit/pedals/saddle/etc did), it was certainly affected by it and probably exacerbated it, even in subtle ways that I didn't realise. My running tempo was perfect and symmetrical (which frustrated my PT 
