Not really ultras, but I’ve done a few LAMMs/KIMMS and the High Peak Marathon (40+ miles) a few years ago, and that has lead me to conclude it’s all in the head as much as the legs (although obviously you can’t be a total sack of spuds). You just need to keep moving, and forget about ‘running’ in the same sense that you might do a marathon or whatever. I wouldn’t really say I’m much of a runner- the longest ‘proper’ run I’ve ever done is a half marathon, but I can do long days out in the hills and know how to get my head down and plod on.
Good advice above too about not trying to sprint up hills, and also as mentioned it’s probably different if you’re thinking about podiums rather than getting round to your own satisfaction. But I’d say the barrier to entry and not being DFL is lower than you might think, assuming a reasonable level of fitness.
Emsz’s point about not running all the way: Definitely agree about marathons etc- if I’d had to stop on my half I’d have counted that as a fail- but this sort of thing is a different game IMO.