I cannot tell you how much respect and awe I have for people who weigh more than I did when I started, because I know how hard it was for me to get started! 15 minutes is absolutely not small when you are just starting out! And look at you, already planning on walking twice more today! I still fit in shorter walks, 10-20 minutes, on days that I just don't have time for anything else. It always makes me feel so much better and less stressed out, and I know I am at least doing something for ME on those days, even if it's only for a little while. Congratulations again on starting your journey! It is amazing how much better I feel now than when I started, not just physically, but emotionally. I am a much happier person for it.
It sounds like you are off to a fabulous start!