My metabolism was in pretty bad shape after a year of breast cancer treatment (the effects of chemo), and lifelong habits of meal skipping. I had my resting metabolic rate tested at my local hospital about two and a half years ago, and was at less than 1200 calories a day as measured by the MedGem. Normal for me would have been just under 1500. At the time I was over fifty pounds heavier - so I was low by quite a bit.
I decided to emphasize getting fit/strong and fixing my nutrition, and not to worry about my metabolism specifically (just to take it into consideration). After two and a half years of hard work - much of it involving Cathe dvds! - my metabolism has been measured again, same person and same test (medgem), and it is now slightly over 1400. When doing the same predictions of normal for my weight, height, and age as for the original, my predicted RMR is 1229.
In other words, my metabolism has increased by 200 calories a day in my late 40s, no less, and while dropping fifty plus pounds, with nothing but exercise and good eating habits. And I've gone from very low to above normal.
All this is is a long way to explain why I don't think it is necessary to focus on fixing one's metabolism per se. It's useful to know what your resting metabolic rate is, just like a scale or tape measure or body fat meter can help you establish your baseline. But to me, the important thing is to get fit, and eat the right amount and kinds of food to fuel your fitness and lifestyle. Not too much, not too little, and making healthy choices.