Standard "average" given for a pair of shoes is about 500 miles. I've had some well over that, some about 400. Generally you start to feel a few little niggles as they approach the end of their life. As you are light I wouold expect them to be absolutely fine for well after the race, however you don't lose anything by getting another par earlier and alternating them with your current pair. Some people say this means they can get more mileage out of them as they have time for the foam to "rest" between runs - I've not really noticed a difference, but I wish it worked with legs!!!!
I think I'd want to have run well over 4.5 miles before race day. I have done a half off virtually no running at all - 7 miles in the previous 11 weeks, and not that much before then. I got round, but it was really rather painful. Don't go mad, I'd try and get a 6 in in the next few days. Do the 4.5s next week, and try to get a 7 or 8 in next weekend. Either repeat that the following week or possibly get a 9 in. Don't do any more than one longer run a week. I've lost track of the weeks, but ease of in the fortnight before the race - just do the 4.5s.
You will not be ideally prepared, but if you've got a couple of 8 or 9ers in your legs you can be pretty confident you'll get round so long as you just take it steady on race day.
As for your ankle - it's not causing you any problems at the moment. Try it. If it flares up, STOP, don't make it worse. there are other races to do when you are properly fit.
