Sure you can lose 100 lbs even without exercise. I did mostly walking and I highly recommend it. If all you can do is 15 mins then great, do it. You will soon see those 15 mins become 20 then 30 then the skys the limit. You will start feeling better and just naturally want to walk more. Don't sweat it, just do it. Follow the MWL Principles/Guidelines/and Recommendations and that 100 will melt off you in no time...just be realistic...it probably took you decades to gain that 100 lbs so dont be in a hurry...just follow the program and you WILL succeed, justlLike I did. See my testimonial link in my sig below and my one year journal link tells you exactly how I did it over the course of a year in detail.
Stay the course and you will be coming back here in a few months a changed person with an entirely new outlook on life!
Good luck and ask all the questions you want...we are a very helpful friendly bunch