I agree with Hoser--your loss looks steady and consistent. Two pounds per week is about the max that is recommended for permanent weight loss.

If you do find it slowing down, mix it up a bit....It is easy for our bodies to get used to what we are doing. Try some interval training on the treadmill. Check your heart rate to make sure you are being effective. Add a little strength training.

Same goes with food--you may need a boost day of a few more calories. (At your current weight, 1500 is a pretty aggressive deficit....your body may start resisting that much restriction by halting any weight loss.)

A final thought....Where are you getting the 600 calorie number for walking 3.5 miles? If I have it right, FatSecret puts walking for one hour at 3.5 mph at 238 calories. I've heard that treadmill calculations are pretty inaccurate, but I don't know much about it. I'm a runner, and the (very) general rule for running is about 100 calories per mile. Maybe there's some recalibration to be done there? It would have a big impact on your net intake each day if this calculation is in fact off.

Just a few ideas. Good luck.