I am refinishing the original floors on a house built around 1910. There is significant surface termite damage to the boards. The structural support was replaced or repaired in the past few years. But for the surface damage, about half the boards have some damage showing. Some boards have lots of termite tubes running along the grain and look pretty bad. No hole or tube is more than 1/2" wide. I'm really hoping to not have to replace the boards. Any advice for filling options?

If you take the time you can make the floor surface strong by filling gaps and exposed tubes with epoxy mixed with wood flour. You will need to sand the surface which may expose more tubes that were just below the surface.
Red oak edger dust mixed with general purpose epoxy will make a good match for your fir flooring. A fifty/fifty mix of hardener and resin is the easiest to use. Mix the wood flour about twice to three times the volume of the epoxy that you are ready to mix. Add the flour before you start mixing.
You may have several mixes to fill the gaps, don't try to mix a large batch because if it doesn't get spread quickly it may start smoking from the heat it gives off as it starts to cure.
Mix a peanut butter consistency and if there is a large hole going through the floor spread it with pressure on both sides of the gap so it won't fall through.
You can top off the gaps within 3-4 hours without sanding and the mix will stick together. It may take 6 or more hours to harden.