By Martica Heaner, M.A., M.Ed., for MSN Health & FitnessMSN Health & Fitness ExclusiveQ: I am severely obese—I am 5 feet 4 inches and weigh 300 pounds. I stand all day at work and by 5 p.m. my knees are screaming. I would like to start exercising, but I don't want to damage my knees. What should I do and how do I begin?A: It's not easy to decide to start exercising when you are in pain and feel physically limited. But it's great that you've decided to do so, since you can improve many aspects of your health by becoming more physically active.It's likely that your extra weight is putting more pressure on your knees, and with an all-day, on-your-feet job, you could be standing in such a way that your knees are out of alignment (either hyperextended—locked out straight—or rotated), which...