What I have found when working with clients is that when they are working out they feel more freedom when it come to their diet - meaning they eat more because they are working out. Make sure you adhere to a well balanced diet and are not consuming excess calories. Try keeping a food journal to see if this could be the problem for your enlarged waist.
Meanwhile, I recommend adding two kinds of exercise to your weekly routine:
1) sweat-inducing exercise - consider interval training. One possibility for a high-intensity interval workout that you can do at home is a 20-minute Tabata workout (I provide details on my website, complete with a few recommended workout routines). All you need is a stability ball and a few hand-weights.
2) a daily core-workout - there are great core-exercises you can do with a stability ball, or, if you're so inclined, 15 minutes of hula-hooping is an amazing core-workout, and a lot of fun!
I'm completely unsure why you've chosen a Yoga Forum to post this particular question. But since this is where it's posted and since my training is as a yoga teacher I'll answer you from that perch.
First understand that the body stores fat in a variety of ways and places. For some people their main storage place is the hips and waist. For others it may be the buttocks. Add to this that it is not possible to spot reduce fat. If your primary storage area is your waist that fat will not be reduced until you slim down the rest of the fat stores. You cannot target weight loss in one area.
Second, the body relies on two things for its shape; the level of activity and the nature of what is eaten/avoided. There is no sound reason under the sun to eat processed foods, foods that are fat-free, foods that come in boxes, packages, cans, et al. What should be consumed are organically grown fruits and vegetables in balance with healthy fats, and a bit of protein.
If you are getting medium intensity activity 4-6 days per week AND your body is not at an optimal weight for functioning, then the diet needs to be examined from top to bottom and adjusted accordingly. A brisk 45 minute walk every day is just enough ONLY when the diet is completely nourishing (read: absent of junk).
And of course from the yoga perspective the physical body does need to be worked just as it needs to be rested and the inner work is to find a deep love for ourselves regardless of our shell (while of course working toward a complete health including the physical body).
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