I will be using overground trains. I have not seen any survey regarding the air quality on trains, but I think its safe to say that its better than Chiswick High road at 18:00. Even if it were worse (which I very much doubt), I will not be breathing hard. The total volume of air passing through my lungs will be greatly reduced.
Regarding the reduction in exercise. Its not as if I'm going to be completely sedentary. I'll have more energy for other forms of exercise.
To be honest pollution is not the only reason I have given up. The fear of being hit has played a large part too. I've survived 5 years with no injuries and hopefully no long term side effects. As I said in my initial post, its like a drug, I love it but I know its very dangerous.