I guess I'm thinking 'metabolism' in an anecdotal subjective sense. i.e. if I don't work out for, say, a month. It takes a while for my system to really get revved up again. Maybe 5-6 workouts later it feels like my metabolism is up, I'm burning more fuel, my digestion is better, I'm sleeping better, have more energy, workouts feel awesome not torturous, etc. That's the physiological experience I mean. When I get into this place my fat comes off fast and it becomes difficult to eat enough to replenish after my workouts (i.e. glycogen and muscle rebuilding).
And the longer someone's gone without being in this place the harder it is to get there and the longer it takes.
there are two types of metabolsim you need to distinguish here. your basal metabolic rate is simply the rate at which your body burns calories just to sustain itself, ie if you just laid in bed all day and did nothing the number of calories you would need to maintain your current bodyweight.
your metabiolic rate as most people understand it is the number of calories you burn on a daily basis for all the activities you do. so yes if you go from a period of inactivity to activity you are increasing your metabolic rate, you are burnig more fuel etc etc simply because you have increased the level and intensity of activity you're doing and your body responds to meet that demand. the reason you feel better is because you're adapting to a higher level of output and establishing a new normal. as the normal becomes higher what was once difficult becomes manageable and what was once manageable now becomes easy.
We don't know what our limits are, so to start something with the idea of being limited actually ends up limiting us.
Ellen Langer