A competitors foot after 50 miles in wet conditions

Running Ultras can be brutal on your feet for obvious reasons.  Your feet can get wet and muddy very early on in a race (especially in the UK) and you need to have worked out a shoe, sock and foot maintenance routine that will cope with this as best as possible.  Inevitably it is extremely hard to stop your feet from suffering to some extent, but its about managing it to keep the discomfort to a minimum.  As far as footwear choice is concerned you need to look at the following:

UNDERFOOT PROTECTION
It may be fine dancing round an hours fell race in your favourite pair of Walsh's, but for long trips you need a shoe that will offer some protection from the ground you are running on.  This is usually through extra levels of cushioning provided but some shoe manufacturers will include a built in  'protection plate'.  This is normally a hard but flexible plastic plate between the out sole and mid sole that protects from rough ground.

FIT
Your foot will swell in size over the course of an event, it is even more important than usual that you have plenty of room in the toe box, otherwise you could suffer in the later stages.  Having said this you still want the upper to offer support in the heal and mid foot so you are not sliding in the shoe at all. You want the upper to be quite soft and flexible as any hard edge will rub over time.

WEIGHT
As you are going for a shoe with more cushioning and protection it will inevitably be heavier than your short race shoe.  However try to get something with a light upper to minimize the additional weight.  Also make sure it won't soak up a load of water and become heavier when wet.

As a final piece of advice, like all of your other Ultra Running kit give your shoes a good testing before you race, so you know they work for you.