With regard to the speed issue.
Yes speed is less important in longer races. Anybody who has done a marathon and a 10k will understand that. Ultras are run at a slower and more relaxed pace than a marathon, but they are certainly not easier.
I can run a city marathon in 3.45 and a 10 k in 45 minutes.
A flat 50 mile race will take me about 9 hours, which is essentially a 4.30 marathon pace.
A hilly 50 miler will take me more like 10.30 to 11.30.
In a 100 miler, I will limp across the finish line just inside the 32 hour time limit.
In all of these races, my objective will have been to finish in the quickest time possible, and these will have been the results.
Having said that, there is a whole ocean of people who could have considerable success in ultras, but don't try one because they are intimidated by them.