In reply to The New NickB:
A you say she seemed to have got the timing right. Most things have already been said, but it's important to differentiate between different types of ultras. A relatively flat 35 miler can just be approached like a long marathon. Anything that requires 10+ hours probably needs a different approach and anything that requires carrying loads of gear and running through the night is completely different again.
On all distances, it's important to eat and drink just right, not too much not too little, but what you eat and how much changes with the time event takes to complete. The potential impact of bad nutrition management also becomes worse in longer distances, although on really long stuff there is often some scope for corrections.
Faffing has already been mentioned. For a mid pack runner like me this probably the biggest factor. Just keeping moving at all time can result in very good performance, probably not a winning performance, but above average. Of course, if you want to compete at the top avoiding faff has to be a given.
One thing I have noticed is that it's difficult to train for ultras, i.e. you get better by doing ultras. There is some sort of physical and mental adaptation that only comes with experience. I don't think I have become fitter in the last two years, but somehow ultras just don't break me down as much as they used to and I can keep moving at my modestly efficient pace for longer with less pain.