Ditch the two-a-days unless your primary goal is fat loss
You will really start to build endurance at the 25 minute mark. A 3 miler once per day will build your endurance faster than two short runs. Stretch AFTER you exercise NOT before. You want your muscles warm and pliable. Find a good stretching routine online, do it religiously, and hold each stretch for at least 30 seconds; 60 is preferable. You should also incorporate at least one rest day per week, two is better. Not having enough recovery time will hurt your gains, negatively impact your workouts, and increase your chances of injury. Your biggest risk when just starting out is tendon/ligament injury. Your cardiovascular strength increases way faster than tendon/ligament strength and too-much-too-soon injuries sideline countless new runners. Bight the bullet and pay the $100 to get some new kicks that are fitted for you at a local running store. The wrong shoes WILL result in injury. I would Google "Couch to 5k" programs, start at the point where you are and follow the schedule. They work. Patience, persistence, and wisdom are the key to becoming a runner.