Slightly longer reply
Ultra running is mainly about endurance, if you can run a fast marathon, you have plenty of strength in your legs to run a slow 60k ultra. The big difference between an ultra and a marathon is fueling, in an ultra trail run, you need to eat pretty much all the time, on a marathon I eat gels, on an ultra I eat proper food, cakes, sausage rolls, nuts, chips etc, I don’t need the instant energy, I need a slow release of energy over many hours.
In many ways, an Ironman has a pretty good crossover to Ultra trail runs as both take many hours and require good nutrition.
I don’t know what the scene is like in the US, however, in Europe, race directors take great delight in setting courses that are rediculously hard, involve using the narrowest and steepest paths, rocks, tree routes, marshes are what they are looking for. The first time I did an Ultra Trail race, I couldn’t believe that they were sending us down some of the paths that they used, I thought that my navigation was off…my navigation was perfect, it was just that the RD was a sadistic B*****d. Now that I have way more experience, it has become part of the enjoyment.
Road runners tend to have a thing about running too much, trail runners know that it is best to walk the up hills. However, you need to train to walk fast up hill, poles make a huge difference. I used to lose tonnes of time on the hiking sections, so its well worth adding hiking into your training.
Running downhill also requires training, you will quickly blow your quads if you haven’t built up strength. Once your quads are gone, running form collapses and you end up shuffling. Luckily its pretty easy to train - find a big gnarly, steep hill and run up and down it a few times a week on a regular basis. I find that quad strengh disappears pretty rapidly, so its something you need to keep doing.
Running downhill on rough terrain also requires a different technique, a forefoot strike gives way more grip on loose surfaces, you need to keep feet higher so you don’t trip on rocks and routes, you need to be confident to “just let it go” as braking/slowing down costs energy. Poles are really useful on steep descents
Someone mentioned triathletes taking too much kit, this is probably true. Normally there is a mandatory kit list, a minimum amount of water, a survival bag, a waterproof etc. I keep my kit to a minimum, and use the lightest kit I can get hold of. If I can get food and water on the course, I use that in my plan