By March you could be looking at 1h45m if your training goes well. That's not too shabby at all.
On the day, I'd have breakfast (toast & cereal) at about 11am, then have some drinks on hand. Don't take on too much fluid close to the start because you'll have to carry it!
Wear your favourite running gear, regardless of what it is. Make sure it's comfortable, though. Get to the start by about 11.45, assuming you've already got your number and chip tag sorted out.
I run in a range of stuff, from proper technical running gear to base-layers that I got from Mountain Warehouse. Most of my shorts are el cheapo Umbro-branded from Sport Soccer. Eww. Socks are important, though, so I use Thorlos or bamboo trainer socks. The bamboo socks are really comfortable, and I've never had any blister problems with them.
As for training up to the event, get a proper programme. There are some on this site.
I expect many people raise money for charity, but don't feel that you should.