I've had the lot: training for London? What charity you raising for? All in one go? I tend to treat all daft comments with a bit of patience, and explain to them the error of their ways. Besides, most mean no harm, they're just a bit ignorant.
I often use the line about how if I run for a couple of hours a day, I'm obsessed; sitting on your arse watching TV & eating for the same period is seen as normal.
Seems to shut them up as well as genuinely offering them a different perspective.
Except for that. That should've earned a swift punch to the chops.
The only thing that really winds me up is if people at work ask if I'm going for my 'jog' when the see me disappearing in my running gear at lunchtime. Jog?! Cheeky mutthafu...!