Iain - check out the website - its 62 miles, 10000ft of climb, most is on runnable paths (but there are places where for some its quicker to go direct), the record is just over 10 hours

You have to carry 100g of food with you at all times, plus most people carry additional stuff to eat on the way round, however there are at least 7 places where food - hot and cold is provided and a couple more drinks only if my memory is correct