I would recommend cushioned shoes, light trail or road.
Food was nice afterwards and had free entry into Stonehenge too - although I was not bothered, but nice touch. Just got the flier through the post today.
Was nice to be close to the car and then home in 45 mins too.
Hold back on the first half as the hills could build up lactic acid quite quickly, but then enjoy the second as you fly along! Or do what I did, blat the first half and crawl the second
.