How do I start?

I feel a really strong desire to make multiple day hikes in nature, without seeing a soul and forgetting my phone. However, I live in the Netherlands, a county with almost no natural forest, no elevation whatsoever and absolutely no wild camping allowed.

Of course nature is beautiful here and I do walk a lot, but it feels insufficient.
My ultimate goal is to thru-hike the Appalachian trail. But how do I go from taking a stroll through the woods with pristine kept paths to surviving in woodlands bigger than my country? I don't know how and where to begin building the necessary skills.