Nice work so far.
Plateaus can be triggered by a number of factors. One thing's for certain... cleaning up the diet by eliminating fast foods and making better choices overall can and will control calories and initiate weight loss. That said, for most, this will be short lived.
An idea might be signing up for fitday.com and purchasing a food scale. Get a feel for how many calories you're actually consuming. Most plateaus are caused b/c you're eating too much. Remember, you can overeat good foods.
It's also important to note that as you lose weight, you have to adjust your intake over time. What was once a deficit will eventually become maintenance as you become smaller. The smaller you are, the less energy is required to move around and maintain the various tissues, etc.
Also, on the exercise front... has your doc commented on what his/her progression plan is? You're not doomed to walking the same distance/intensity for life, right?