Not sure why but I just don't like the taste of it. If I've been drinking normal Coke and then switch to diet then it just tastes wrong for ages. After a while I get used to it but I still can tell it's diet. Worryingly I can tell the difference between Pepsi, Pepsi Max, Diet Coke, Coke, Diet Caffeine Free Coke and other

Have you tried swimming? That's quite good exercise and it's low impact.