Newly purchased house - Hardwood floors stained dark in some spots

Hi everyone,

As the title says, recently purchased my first house (yay), but while I initially noticed these stains in the floor during a daytime walkthrough, I realize now they're a lot more bothersome when it isn't super bright inside. Images in the attached link

I'm not moving in for a few weeks, so wondering if I can fix this beforehand (I know for a fact it won't happen at all if I wait until after I've moved in...). It's mainly this one section in the hallway and one of the bedrooms, as shown in the pics.

I did some initial research and it seems that they're likely going to need to be completely refinished, and I had a few questions about doing that myself (or would love to hear if there are other options!).

1. How easy of a task is this for a beginner to woodworking etc.? Most of my DIY has been in the realm of tech, so I don't have any experience or needed tools. Maybe I'm in over my head here and it isn't something I can do myself.
2. What tools/equipment do I need? I believe just a sander (any recommendations would be nice), wood stain, and sealant. Plus brushes to actually spread the stain/sealant.
3. Can I just refinish the areas that are stained, or will it look horribly uneven if I do that? Do I need to refinish all of the flooring if I do some? (80% of the floor is this hardwood floor throughout the house, there's not a break in it or separate sections)
4. Given the answer to 3., about how long would such a task take? I assume there's a lot of passive time waiting on stain, restaining, sealing, etc.

Thanks for any help, really appreciate any advice!