Arthritis is very common with back problems, but that doesn't necessarily mean a whole lot of pain. I picture a little old grandma with permanently curled fingers when I think of arthritis, but really, there is a wide range of effect. I was told at age 18 that I probably had some arthritis in my back after my first fusion. I was devastated and cried buckets of tears anticipating that granny type problem. It never happened. It just meant a little extra stiffness and achyness sometimes. I never took anything for it at all.
I'm expecting, especially with my second fusion now, to have an increase in that stiffness as I get older, but I don't plan to let it slow me down any more than necessary. When the time comes, there are lots of medications for it, and I'm happy to take them if they help. It's a shame the doctor mentioned it but didn't take the time to give you more information. I'm sure there are some people who have it to a very painful degree, but for most of us, I do believe it's something we can take in stride with the rest of our back issues.
I don't mean to brush this off. I just don't think it's something to worry yourself about at this point. It sounds like you're really doing wonderfully! Take care not to overdo it. Sometimes there's an increase in pain after the first couple of weeks of recovery, but you'll get past that and be back at life before you know it! Just don't rush it. Your body needs time to heal, even if you're feeling great!