Mountain Marathons generally involve off-road, off-path terrain - grass, rocks, bogs, tussocks, heather etc! As such they require proper fell shoes. Road shoes should never be worn, as they have insufficient grip and can even be dangerous. Road / Trail shoes generally lack sufficient stability, as they hold the heel to high from the ground, and allow too much movement inside the shoe. Trail / Fell hybrids may be ok in some situations, for example in some summer MMs when the ground is hard and firm and some cushioning may be required. However the winners choice will always be an out-and-out fell shoe, provding grip, lightness and a tight fit.