Hello and thank you for your question.
I am sorry you waited so long for a response. It seems your question was in the wrong category and was recently transferred to the correct one.
Exercising 4 days per week is very important and is a great start. Your cardio should be at least 45 minutes followed by some weight lifting. Prior to cardio you need to do about 10 minutes of light stretching. Due to your back issues, the best cardio for you would be either a stationary bike or swimming. Swimming, in general, is one of the best exercises as it works all muscle groups and allows you to burn the most amount of calories.
In terms of food...one of the most important things you must do if you wish to loose weight is to cut out late-night eating. Everything you eat late at night is stored as fat and nothing is used up for energy. Your most important meal of the day is breakfast which should consist of mostly protein (eggs, cottage cheese, turkey bacon/sausage). Your dinner should be the smallest meal of the day and should not be later then 6:00 pm. If you get hungry after 6:00 pm, you can have a protein bar for a snack. Also, try to cut out as many carbohydrates as you can (bread, pasta, potatoes, rice, pizza).
Based on your weight, you should consume about 1500 calories per day in order to loose 2 lbs per week. Once you achieve your goal weight, you can increase amount of calories to 2000-2500 per day for maintance.
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,