Perhaps if you are walking for an hour a day you can sustain the 1200-1500 calories a day. Otherwise, it is probably too much. If you are stalling you might try shaving down your calorie count till you start to lose again.
As far as only being able to walk for 20 minutes at a time, I have found when I first start to walk that after 20 minutes my hips and back hurt as well. When I sit down for a few minutes and take a rest, then get up and walk more it seems to help. I have found the best bet is for me to walk on a trail that is 2 miles long with a number of benches and stumps I can stop at and rest for a few minutes before I continue on. Because the trail goes through the woods it really helps me force myself to keep going. You can't get out of the woods to go home till you walk out.
Try going for a walk outside so when you walk 20 minutes and get tired you can rest then walk back home. You will find that you will build stamina, and can gain more ability.
Good luck, and keep us posted. I haven't started walking yet, but I need to. My surgery is 10 days away. The snow is deep outside, but I think that I will head over to the field house where they have a track you can walk indoors on. Living in Alaska in the winter poses some of it's own challenges for walking outside in the winter.
Most importantly have some understanding friends or family walk with you. Their support will count for measures.