I began my journey much heavier then you. I started walking 15 minutes at a time, 4X a week. I did notice I was getting results and strived to keep on increasing that. I quickly went to 18 mins, then 20 and 23 and so on. I also began doing it 2 X a day. I figured if I couldn't go longer at one time, that doing it 2X daily would benefit me, and it did! That's something that you may want to consider.
I think it's important to get in some sort of activity most days of the week. There are days when I used to just crank up the radio and dance my fool head off.
Keep at it and you'll be amazed at just how quickly your stamina will increase. Good luck.