In Night at the Museum Robin Williams states that ‘the secret to happiness is physical exercise” There are now plenty of studies which have found connections between feelings of well being and movement. This doesn’t make it always easy to do. The answer very much is to follow our ‘gradual infusion change.’
Also, think of it as movement, not just exercise. There are 3 types of movement: general activity (i.e. not sitting down!) moderate exercise and intense exercise. Start by choosing just one to increase. Or maybe start by recording how many hours in a day you sit down for: it may be a shock!