1. No consciously lengthening your stride will increase your risk of injury because you will be overstriding. Your feet should land close to your body, not far out in front. Otherwise you will be increasing the amount of time you spend in the air, which reduces your efficiency and you will land on your heel, which acts like a giant brake and sends shockwaves up your legs. As you get fitter, your stride length will naturally increase in line with improvements in your racing pace.                 

2. Concentrate on your diet. Unless you're running more than 50mpw, running doesn't provide much of a stimulus to lose weight. That being said, each extra mile you run is 100 calories burnt, so increasing your mileage could help.