I agree with Ultimate that it's time for a break from impact sports.  Look into pool running, which will preserve your aerobics while giving  that ankle a chance to tighten up with less internal scarring. Bad  healing can result in impaired circulation and nerve damage, but a break  with proper rehab can get you where you want to be, faster. The half  marathon, even if completed, may delay your progress. Trust your ankle to heal. I've twisted mine badly within the last few years, and recovered.

I am close to your age and know the impatience thing.  As we age there is a temptation to think we can only get slower, but I  know guys in their 60s who are getting faster. The important thing is to  stay injury free, and to manage injuries constructively when they  happen. I used to race constantly. Even if I had to limp through a race I  would compete, but I've learned the patience to play a long chess game  with my body, training sometimes for several months between  competitions. The healthy body wins over the compromised one, every  time.

A little over a year ago, I started a marathon  after training hard for a year. I cramped badly within a couple miles  and dropped out. It was the first time I did not finish, even though I  could have toughed it out. A few months later I ran another one with  some of my best splits ever, and a year later I had my second best time  to date, the best being a few years ago. Patience can bring rewards, no  matter what your age.

You've come a long way and have a lot to be proud of. You can preserve it, or jeopardize it as you wish. I went into my last marathon with a slightly sore hamstring that continued to bother me for months, so I took time off from running. I only completed about 30 miles over the last month, massaging my hamstrings daily. I actually thought I was going to die a week ago, after only a half mile of huffing and puffing ended a week of rest.  The next day felt better, and a few runs later I feel great, as if I  hadn't lost a thing. Sore hamstring - gone. I challenge you to enjoy  your recovery, stage your comeback, and chop an hour off that marathon within the next year. I'll bet you can.