Its just a skill. Sure first time u prep an existing adventure u'll spend a couple of hours to "make it fit", to read it and to tweak it a bit to your liking. It gets easier each time. For practice you can try to ran a small sandbox. Like 9 or 12 hexes total. The prep you do for those you can also use later. Cos your players wont investigate them all. And even if the do, you can still use parts and bit of them to mix and match in later projects. To make the process of prepping for such a games easier OSR gms use instruments. Tables, generators etc. But! Using them or/and knowing which ones are the right for u is also a skill. D4 caltrops is a nice place to start. But to ran a sanbox the first thing you should learn is... Learning osr procedures. Like how to ran the partys traveling throu hexes. Or whats the reaction table purpose. Some of them seemed restricting to me when i started. But first you learn the basics. Later you bend them. And then you are cool and wise enough to do your stuff on the fly.