Can I lose 20 lbs by the middle of September just from walking everyday and eating healthy?

You can lose weight by creating a calorie deficit by the end of everyday. You'll have to burn 3500 calories to lose 1lb. That's a calorie deficit of 500 a day. Not eating is the wrong answer. Eating more but the right stuff will keep your metabolism burning calories and lower insulin spikes which tend to cause fat storage. Resistance training also speeds up fat burn up to 24 hours after you work out. So you need to work that into your program. I would also suggest some intense cardio. Not jogging or walking but sprinting for example. Yeah it will be tough in the beginning but like anything with time gets better. I found a whole bunch of great ideas at bestabsdiet.com