finishing wood floor area to match existing

chongololo

Greetings folks.
Here is my scenario. I have a 1952 house with wood (oak?) floors throughout. It looks like carpet was installed from the start and then ripped up at some point. The floors were then sanded and finished probably within the last 20 years or so.
In the living room they installed a pellet burning stove in the corner on top of a slate tile base which in turn was put right on the carpet!
Long story short, I have removed the stove, the tile base and the carpet. I'm left with a section of floor that is unfinished and slightly higher than the surrounding area.
The levels can be taken care of with my belt sander but my question to you folks concerns the matching of stain or clear coat.
I'm concerned about the new finish not matching up with the old. How do I 'feather' it in to match? Could I run the sander over the old finish a little ways or will it gum up?
Any suggestions to the type of finish to use for the newly finished section? The existing finish has a slightly yellow/light brown color to it. Around the edges of the area of previous carpet, it has a glassy brittle flakey feel to it almost like dried tree pitch.
Any tips you would be willing to offer would be greatly appreciated.
Thanks.
PB