"Just wing it--you'll be fine."
While that may be too glib and doesn't provide any specific advice, I applaud the general "less is more" sentiment. The only problem I see with "winging it" is when the winger doesn't understand what makes the game fun for their players. If the game starts to feel like an endless array of random encounters with no particular stakes, then that's not going to work for me as a player. But if you're putting pressure on my character, building off of previous plot threads, following your players' lead, and introducing meaningful stakes, you don't need to spend hours starting up NPCs and monsters and drawing meticulous maps. I'll be having so much fun that I won't notice (or care) if you whiff some details.
Meticulous planning might be fun, rewarding work, but it is certainly not necessary.