For me rides of 20-30 miles are all on local roads that I know, so no need to plan, although sometimes it might be beneficial to actually plan a ride before I set off, especially when I'm limited for time
Quite often I'll head out with a rough idea of a route in my head & then change it as I'm cycling along, thinking this road will add a mile, that road will add 2 miles, but quite often I underestimate the added distance & the time taken. I was nearly late for work last week doing such, what I thought would add 2 miles actually added 4 miles, so I was then pushed for time
I can go out onto the wolds for 100+ miles without a planned route, but if there's somewhere specific I want to go, a certain climb or velo viewer square then I will plot a route to ensure I get there, but then often change it at some point 🤔