Hi Roger, we have an annual parish walk here on the island, where you have to walk 85 miles in 24 hours, and I can tell you now - wear trainers!!!!!! 75 miles in 20 hours might not sound fast, but believe me having to keep up your speed for hours on end can be draining.
Assuming you are fit, the biggest problem you are going to have is blisters, so get your feet sorted in as many ways as possible, including breaking your trainers in, getting the right socks and applying compeed / tape BEFORE you set off, and as soon as you feel a heat spot, stop and sort it out - its no fun walking fast on shredded feet.
Its best to have a support car who can meet you at designated spots to supply you with food and drink, but I also use a camelbak with some of those resalting packets in to help keep your body in shape.
And make sure you can be seen at night, especially if there is no street lighting on a particular stretch.
Google "Parish Walk" and check out what people are wearing, I'm the one in the trainers...