I answered
Slow down to their walking speed and keep the same distance

I could see myself doing any of the three options, depending on the circumstances such as how far we are from the end of the hallway, whether I'm concerned about getting somewhere on time, whether there are other people around, etc. I'm also not sure how I would be able to judge that the person is walking at 90% of my walking speed; usually it seems like it's closer to 50%.