Shoes
For a runner shoes are naturally of special importance. Each runner has a individual formed foot and must therefore find the right shoe. Most athletes are pronators, which means the foot turns inward in the movement. If you turn your foot outward, this is called supinating.
You can find which type you are easily by looking at one of your old shoes. In case of strong pronators, the inner part of the shoe sole is worn out, while for strong supinating it is the opposite - the border on the outer back of the sole is more worn. Runners with normal foot movement wear out both sides of the sole equally. It's important to find out what kind of runner you are, so that you know what kind of foot support you need to harmonize the movement of the foot.
As the foot swells during multi day races, it is also good to know that brands also offer different widths as well as sizes - for example, extra-wide shoes are noted as 2E or 4E (normal shoe width is denoted as D or sometimes B).
For races that are lasting for more days, you need to have shoes ready with sizes up three numbers bigger then your usual size. I still remember my 700 mile race (1994), on which I went to the start with three pairs of shoes. After the second day already all my shoes were too small and I had to torture myself for the remaining days with those shoes and blisters became my best friends.
During a race, many ultra runners mutilate the shoes with knives and scissors to deal with blisters and other problems, sometimes at the end they look more like sandals than shoes! To swiftly solve shoe problems during a race, you absolutely need to have creativity and spontaneity.
Socks
The socks play an essential role. There are many different models on the market, and it is important not to try to save money here. In my case thin, fine vowen coolmax socks turned out fine. I get easily warm feet, therefore the socks cannot be to thick and must easily transport the humidity. It's also important that the band is not too narrow, because liquid could collect easily and provoke infections of the skin.
Clothing
"There is no bad weather, there is only wrong clothing" - There is a lot of truth in this expression.
Coolmax, Transtex, Goretex …cost some money, but if you run for hours you learn to appreciate the better quality. Cotton t-shirts are comfortable to wear, but once you are soak with sweat it takes a long time to dry up and you cool down very fast.