Helen - what a fabulous story of courage. 56 is hardly too old to do a marathon, but training is everything, I think.
I did my first running marathon on Bougainville Island in PNG. As I read your story I thought that perhaps I should write mine. I was a little younger than you, 40, and regarded by the younger participants as the granny of the race.
The mind is your greatest strength. Self-talk is everything. Someone on your tail, someone you are not prepared to lose to, well that just gets the adrenaline flowing, even when one feels there's nothing left in the tank.
good story. Good cause.