How I stay fat is that I bring my whole attention to my work to make it as easy as possible.
For example, while working every footstep is calculated; I never go back to my van to get something I've forgotten - I forget nothing - therefore I never use unplanned steps or go over budget with my walking allowance.
I reckon this saves me at least three miles a year walking, and when calculated over the long term (if I live that long without contracting coronary heart disease), I reckon I'll have shaved of 30 miles over the next ten years.
I'm the same with exercising my dog; I use a 'thrower' it (saves bending (something that never bothers Squeaky) and makes throwing the balls physically less demanding) and three balls. My dog never brings the balls back to me, so each round of making him run after said three balls involves me doing Pythagoras type calculations to ensure that he does more mileage than me.
Simple really.