I do enjoy long walks, though the longest I've taken myself only lasted four days. That was without purchasing anything though or making stops, so you could probably do with carrying less food than I did. You also probably won't need to bring a guitar, but I don't go anywhere without my guitar. I'm lucky a sixty pound pack doesn't slow me down much.
A few things to keep in mind -
Consider backpacking in general instead of walking. There are some places and times when you'll really want to hitch a ride. Don't be afraid to use public transport when in a city to get get around.
I'd recommend at least a light sleeping bag or blanket and tent rather than a generic tarp. There will be stretches where you'll be sleeping outside.
For backpack, go with a high quality large military-style backpack, with a frame and bits for attaching things.
When traveling through the countryside, don't be afraid to ask people if you can pitch camp in their fields or property. It's a lot better than being caught trespassing, and sometimes they'll invite you to have dinner with them.
Carry plenty of water on you, especially in the SW. Get some better advice from someone else about hiking in the desert.
Budget for at least one new pair of shoes over the course of the trip. You WILL most likely destroy yours earlier in the trip than you expect.
Bring a compass and a manual recharge flashlight. I prefer the crank ones.
If you plan on going into the wilderness, consider boosting your phone or getting an emergency radio. It's not light, but it could save your life. Better yet, stick to the roads and map out your route in advance so you don't have to worry about it.
I don't go anywhere without some rope, a good knife, my folding shovel/pick, my travel cooker and a small pan, but you probably don't need these as much if you're sticking to civilized areas.
I'm not sure what you need the satchel/hip bag for?
Unfortunately, except for sleeping outside, finding a place to stay is going to be a problem. I've spent most of my time hiking in places where you can spend seven dollars for a bed for the night, but there aren't a whole lot of places in the US that are like that. Look for youth hostels if you can - I've seen beds for as cheap as 14 a night in the US. But don't expect to find them. Even staying at the cheapest motel is limited due to the fact you have to walk to it, so I'd budget at least 30 dollars a night for accommodation.
And I'll join the chorus of doing a still long but also much shorter walk first. A week or even a two week walk will provide you with a much better understanding of exactly what this means, and help you prepare better.