alt Hacker NewsOSR systems have a ton of already published modules and campaigns to draw from, if you want zero prep (beyond reading, and maybe drawing out maps or something). Otherwise, it's about the same as running any other system in my experience. Easier than some, harder than others. The Dolmenwood campaign provides a lot of information for the DM, too.
At least for me now, I'm quite older than I used to be, and I don't want to have to read a lot of information or setting details ahead of time to be able to run a game as a GM.