Phil - First up, congratulations on achieving the 10k. That's a great achievement.
I have a syndrome as opposed to having had an accident but life's had to change a lot too. My concentration and memory have been greatly affected.
I find like exercise, the less I do the harder the worse my concentration gets so I try to make plans with scheduled breaks if I'm actually trying to concentrate and days off. Though even that is sometimes too ambitious. I've even had to rewrite these paragraphs about 10 times because I'm still not sure it makes sense 
For the down times, can you still see colours okay? A friend suggested Mandala colouring books to me. The pictures have patterns in them and though I don't love them and I'm not an arty type, it's just another way of doing something.
Edited: 17/08/2012 at 10:06