Hi Sam
I have moved your question to a post of its own so it doesn't get confused with the other answers on the original post.
Only you can decide whether or not to appeal. What are the distances like in your home, and do you move from one room to another? If the distances are less than 20 meters, you can send them pictures and distances to refute that assumption.
I googled how far a person can walk in 15 seconds, and it came up at about 90 feet (27m). I'm not saying this is right - but I'm sure they use something similar to make their assumptions, and this is why you have been put in the 20m to 50m distance. This doesn't mean you shouldn't refute it - particularly as you guessed how far you could walk, and it's unfair because unless you've been timing yourself regularly, you have no idea. If you put in for an MR - you need to say that you had no idea. You should also emphasise that you need to stop, the effect of the pain, the safety aspects if you're trying to lean on crutches or against a piece of furniture and whether or not you could repeat the journey (which I doubt it).
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems