Consider the Camino de Santiago. I've done the last 300 km. The distance between places to stay is manageable, lots of infrastructure, lots of friendly people who could help you if you get stuck. You don't need to carry much because you sleep indoors and there is food available along the way.
The next step would be doing a hut/refuge to hut/refuge hike in the Alps. A hut/refuge is a hotel with a restaurant, private rooms, dorm rooms. They are hundreds (thousands) of them throughout the Alps. You can arrange them yourself for multiday hikes. There are an infinite number of routes, but a good starting place would be this publisher of guidebooks: https://www.cicerone.co.uk/.
Likewise, you only need to carry your lunch, but you do need more clothes because mountain weather can be unpredictable and dangerous.
I've personally done the "Tour de Vanoise" which I think is easy enough for novices and the "Stubai Hohenweg" which was definitely harder and scarier.
Actual backpacking (tent/sleeping bag, etc) is harder because you need more equipment but a good way to start in the United States might be by doing a group trip with The Sierra Club or a local club (meeetup.com) or commercial trip (REI?). I've done many Sierra Club trips.