Just out of curiosity, once you get out there with your backpack, what do you want to do?
Do you want to hike a lot of miles?
Do you want to hit several parks and spend a few days at each?
Do you want to get into the forest and find a cool spot and spend a week or more there and not hike a lot?
All of those can be a lot of fun.
But... You know, 28 days is a long time to be mostly all alone, especially for someone your age. And you are going to be a long way from home, with no wheels.
I have to tell you, I like your style. Cars and gadgets will be worn out and forgotten in just a handful of years, and the memories of your trip will last a lifetime, but 28 days in the Oregon Forests may be overreaching a bit.
Consider how you'd hold up during a solid week of rain. Or even worse, snow.
I like aimless's angle of "finding a trail-rich area not too far from a town and exploring it in a series of moderate-length hikes with resupply runs in between them
Exit plans are really important. If you get weather of epic proportions it's really nice to be able hike out in a few hours and dry out, warm up, and hunker down in a motel. That's good planning.
And I'll also urge you to practice using your gear. You can do that in your back yard. Definitely practice setting up your tent in the wind and rain and spend a few nights in it while it's raining too. It rains in Oregon, you can be sure of that. And it sucks to learn all the ways a tent can leak that are your fault when you can't make a fast dash back into the house