The best advice that I can give you is to take it as slow as you need to and listen to your body! General pain and discomfort during and even shortly after your workout is completely normal but sharp pains or ones that don't go away fairly quickly afterwards and your body is trying to tell you something. I started overweight at 265 and learned this lesson the hard way myself. Everytime I tried to "suck it up and deal" or just "push through it" like people (usually in better condition than us) will tell you to do I ended up sidelined with an injury that took weeks or months to recover from causing me to have to start over. It's easy to get too eager, for me I've got a 9 year old daughter who is running 5K's and wants me to run with her! Just take it easy, celebrate all of your accomplishments, don't compare yourself to anyone else, and most importantly remember that you're still beating everyone on the couch! Good luck with your journey!