You're out of shape and your body is not functioning efficiently when exercising (or in general for that matter). Don't be too concerned with your heart rate. remember that everyones body is different and responds differently to similar exercises.
Just keep walking on the tread mill. As long as your heart rate isn't skyrocketing there's no need to worry. Your body will become more efficient as you continue to workout. Remember not to get stuck on the treadmill only! Weight lifting and diet are gonna be important with your weight loss as well. Switch up from walking, to biking to erging if you can