It is perfectly reasonable to lose 20 pounds by May! Keep a positive attitude, just remember that of all things!
I dropped 22 pounds from March to August. I followed a diabetic diet, which is similar to Weight Watchers, and made sure to work out for an hour at least three-four times per week with a balance of cardio and weights. No carbonated drinks either. Strictly water or tea and one glass of milk per day.
Here is the key to remember:
3500 calories = 1 pound
To maintain your current weight, multiply it by 10 and that is how many calories per day you need to maintain.
Example - I weighed 150, so to maintain my weight, I needed to consume 1500 calories per day.
So to lose one pound per week, I shaved at least 500 calories off per day, putting my caloric intake, before exercise, at 1000. As you lose pounds, you redo your math. And if you want that piece of cake or ice cream, just do a bit more exercise that day, no big deal!
Counting calories and such at first is tedious, but it becomes ingrained in you to make informed choices on what you eat. A great Web site that shows calories for just about anything is calorielab.com.
But now I have a whole new lifesyle of eating and I love it. In a typical day at work, I eat reduced sugar Frosted Flakes with skim milk, fruit, pretzels, a roast beef or turkey sandwich and yogurt and then have a sensible dinner when I get home.
The key is to make your diet something you can make a long-term commitment to.