Option Please
For certain aspects of my job this would be great, others not so good.
Writing will get worse on a tread mill, I have taken work and made notes on it at the gym on a treadmill (at a fast walk). The handwriting is only something I would be able to read myself. I think typing would be possible and relatively easy at a slow to medium walking pace. I have also made notes on work on a seated bicycle, these are much more readable, but pace suffers while writing.
Reading, even critical or technical reading is possible in either situation, in fact I found my concentration improved. Of course that could be because people interrupt me all the time at work and only a few people would do the same at the gym.