Hi Waxwing
I have come across quite a number of claimants who have no concept of distance, quite often they will exaggerate the distance they can travel.
I use bus lengths in measuring distance where one bus length = 10 meters, I find the concept easier for claimants to grasp.
In regards about time to walk a set distance, time them how long it takes to walk from one point to another such as their front gate to post box at their normal pace, do they have to stop and catch their breath along the journey, then time yourself to walk the same distance at your normal pace.
If they take twice as long as you then explain this on the form, could they repeat the journey?
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
