omar, I am not sure this is as crucial as you say! I am very portly (as Rebekah has seen - but I was even more portly when I started two months before she saw me!!!) However, even though I am portly, I had done quite a lot of walking before I started, just on the hills near the city where I live. I had been concerned that I hadn't done enough training with a large pack on, but in the event, it was enough, and I was able to take the initial hills up and down out of Le Puy in my stride. What was the hardest thing in fact was the blisters I got from days 6-9, when I had three days of solid rain and my feet got saturated. In hindsight, one of the best things I did was to take a day off in Estaing to give my poor feet a chance to lose some tenderness.
I saw quite a few others walking who were as overweight as I was, or even more so. I think that as long as people do listen to their bodies, as Rebekah says, they can complete the Camino. I was quite a 'tortoise' along the route, and walked only as far each day as I felt comfortable. I think that anyone with a goodly dose of determination- enough to see you through the harder patches when the way seems too long and things don't seem quite as rosy- can make it, so long as they have allowed themselves enough time. (And so long as they don't fall prey to an accident or serious illness etc.) To me, willpower, and 'wanting' to walk, is far more important than losing weight.
Margaret