Check with your doctor first, always. Especially if you, like me are out of shape. Baby steps are the way to start your road back. I recently met a lady at the Solana Beach Triathlon that could only walk 150 steps when she started getting back in shape. She completed the triathlon and a couple of marathons. Find something small to start. I used to go try and run like I was in high school and weighed 150 pounds again. Many injuries and false starts later, I’m finally a runner. The key is to do just a little 3-5 times a week. Walking for 5 minutes doesn’t seem like much, but if you are out of breath and feel like your heart is pounding. That is a start! My advice for those of us that are morbidly obese is to start with what you can.
Side note: you may be sore for a while. Depending on how hard you push, you may be sore for a long time. I was sore after almost every workout for about 6 months. Motrin and ice are your friends. My knees have been my excuse for not running for years. The crackle, not a little, a lot. They creak and they ache all the time. They used to anyways. Over the last 18 months, my knees stopped hurting as much, and then almost at all. The muscles around my knees are more built up now (not big at all, but strong). That and I lost 60 pounds. So yeah, start somewhere.