I started with 10 minutes and added a minute a day, sometimes 2. At about 20 minutes, I started increasing the speed. Then I alternated increasing the speed and the time. I have a bad back, so cannot use the incline as it adds stress on the back. I was up to about 40 minutes a day walking faster, but then a bulging dicc started pressing on my spine. Now, I am lucky if I can tolerate 5 minutes on the treadmill. I am getting a corizone injection on Monday, so hopefully I will be able to start walking more. I'll probably do the same thing as before, increase the time and speed slowly.
Also, resistance training or weight training is great to start as soon as you doctor allows. Again, start slow. With my back issues, I've been unable to do weight training. Can't wait to start again.
Good luck and listen to your body. You want to push yourself without hurting yourself. Weightloss is not a race, but a journey.