Hi JKSSAPHIRE, and welcome to the board!
What you had was a 360 fusion. An ALIF is the anterior only approach, that is, through the stomach. PLIF is posterior only, through the back. I had what you had, through both the stomach and back. You're very early on in your recovery. Some of the surgeons have a bit of an unrealistic view of what your recovery will be like. Back to work in only a few weeks is unrealistic, unless you have a desk job and a cot beside it to lay down when you need to, and a boss who won't care if your pain meRAB make you too goofy to actually do any work!

For most of us, driving permission was around 6 weeks. I didn't want to drive even then, and only did so when I had to, like to drs. appts. This was a major, major surgery. It is comparable to open heart surgery in difficulty of recovery. I'm not saying this to discourage you. Please, I've been there. It's so much easier if you know that where you are is normal! It can take a full year to completely recover from this surgery. You'll be up and functioning long before that, but you'll need to take it easy. If you need to get back to work, you might be doing nothing else for a while except working, then going home to lay down and gear up for the next day. If you're able to go back part-time at first, that would be great! But don't demand too much from yourself. Your job right now is to recover.
Note: I had a long fusion, 8 segments for a total of 14 from two surgeries, 30 years apart. Your recovery will probably be a bit easier than mine was, but there will certainly be some similarities.
I was told to wear my brace any time I was up, except in the shower. If I got up to use the bathroom and was going right back to bed, I was allowed to not put it on. Someone here had a great idea for padding the brace against that sore incision back when I had my surgery. She used feminine paRAB on it! Gotta like that creativity!

I was allowed to switch to a soft brace at about 2 months post op, but I didn't want to. Long story. The hard brace was better for me. I was allowed to start weaning out of it at 5 months post op.
Walking? Yes, absolutely! You need to walk to get bone growth going. Walking gets blood circulating around your spine, which is essential for bone growth. No treadmill, unless you're slow and it's not inclined. Natural walking is best. Even if you can only walk for a short distance, that's okay. Do what you can tolerate, try to increase it a bit each day, and then rest. Walk and rest, walk and rest, walk and rest. Don't over do it. Okay, you WILL over do it! We all have. You'll pay for it by having increased pain for a day or two. And you'll learn to listen to your body.
Why did you stop your pain killers on a regular schedule? Did your doctor tell you to? If he's prescribing them, you're better off taking them, even if you have times when you feel you don't need them. You'll heal better if you're not using your energy to bear through your pain. Do you know not to take NSAID pain relievers, like ibuprofen? It can interfere with bone growth.
It's very normal to have ups and downs for no apparent reasons with this surgery. Sometimes you can pinpoint why you may be having a more tired or painful day, but many times you can't. It's just your body trying to heal. If you have a harder day, that's okay. You just need to rest more that day.
I hope I haven't made things worse for you. I do better when I know what to expect. It helps me to, as you said, "get my head wrapped around it". If you have other questions, please do post them. No question is too big or too small. We won't sugar coat and we won't make up answers. If we don't know, or if it's something that a doctor would be better answering, we'll say so. We're all back patients, too, and we understand.
I wish you the best as you continue in your recovery. And by the way, I'd do it again in a heart beat! I still have pain, but nowhere near as bad as it was. My doctor is my hero!
Blessings,
Emily