Yeah, then just find couple of those spots you would like to visit and route your backacking through those. The goal is to get form one to the other, while finding camps on the way. And then you just need to start from the first one...

nout

The country will be The Netherlands, which doesn't have mountains, but I've been thinking about "connecting" the major parks somehow, too.

I've used alltrails a few times, it's okay, but has led to a pretty big fail once. 

Fabs

earth

How would you plan an extended backpacking trip?

Find a natural park in some mountains, look at alltrails or similar app, find some cool hikes and find a trail between them, depending on how much time you want to spend. 
Things you want to stop by are lakes, cool vistas, waterfalls, historical buildings...

If you are in Netherlands, then just take train to another country that has mountains 😅