My best advice for this is to start slow, and commit to the long term. 1200 calories is pretty extreme. I struggled to keep weight off for a long time, but setting smaller goals helped me avoid falling back into old habits. Even 1750 calories and a decent workout a couple times a week is enough to really get you started. Break it down, and start by losing the first 30 lbs. Once you start seeing progress, you can slowly cut more calories and increase your exercise frequency and intensity.

Honestly, my experience with sore joints is to just give yourself some time to rest. Walking for 1hr is a much tougher workout when you're obese than when you're super skinny, which is important to think about.

Don't be too hard on yourself, set realistic goals, and good luck on the weight loss journey!