As just a straight 1:1 translation to 5e, it isn't really hard. The monsters are roughly the same, so it isn't hard to run an AD&D adventure on 5e. I would argue a "novice" DM could read through an AD&D adventure and do the translation on the fly. (EDIT: When I wrote this I meant look at the monster in the adventure, and find the analogous one in the 5e monster manual, and use that. Don't use the original stats!)

HOWEVER, I would offer that from the TSR AD&D adventures I have run, AD&D is a) much more "hack and slash"/murder hobo like, and b) much more deadly/much more "random", as compared to a WotC 5e campaign. Not to say that is bad, but I find that if you want to do it, you will want to tell your players this so they have the right expectations going into an AD&D style campaign.

I personally (as a DM) prefer having (some amount of) plot line in my adventures, so I find myself filling in a lot of gaps myself if there needs to be a plot line (which I personally enjoy). I also prefer not to kill my players characters (granted, I also level set that doing things you expect to get you killed will in fact get you killed), so I tend to tune back a lot more of the "deadly" traps (i.e. in some of the really deadly dungeons, just a bad roll of dice on a failed trap will kill you. I am not a fan of that, so I tune it back that it will eventually kill you, but you have a chance of reacting and your teammates can save you).

But I will also offer, I have had players who want to run it "old school", so I don't do any of that and run it as a 1:1 translation, and we have had a lot of fun with it too!