I write this assuming that you already have a good pair of running shoes provided by a running shop.
A podiatrists made insoles for me a few years ago. Prior to that I was always plagued with niggles and pains. Then a bad injury picked up during a maratrhon stopped me running for over 3 months. My problem was also overpronation.
Unfortunately these insoles are not cheap but they have certainly worked. It was worth the cost to me because my running became enjoyable (as it should) and I'll carry on running for a long time (hopefully).
Your physio should be able to recommend a good podiatrist. Give them a call to find out how much they will charge (consultation costs and insole costs) and make sure these charges include any after-sales service that may be required for alterations that are necessary after you have run with them for a while (they will feel strange to begin with).
There are different types of insole (hard for shoes and cushioned for your running shoes). I have cushioned insoles that can be transferred to my different trainers but don't use or need them in my shoes.
Perhaps we should all arrange to meet up at the start line. I'm with Fatbutfit!
Remember your carbo-loading before the run is important but probably won't get you through to the end on it's own. You must ensure that you start off hydrated and in good health. You must keep hydrated through the run plus make regular use of the Sports drinks and/or an energy gel.(I'll be using an energy gel belt)(Note: important to try these before the race day)
I'm not an expert - just run the FLM a few times.
One thing is for sure - you'll have a great time however painful it gets.
p.s. things to remember - plasters, bin liner, spare toilet roll, vaseline, smile.