Don't worry about cutting your workout short, always try to remember its quality over quantity. When you are working on speed, you especially want to warm-up. For your other normal, base mileage building runs, you don't need to spend that much time warming up since your pace will not be as fast as a speed workout. Generally most people agree that running on a treadmill is harder than running outside. There are many reasons, but definitely when the weather is cold, a lot of people prefer to run indoors. The good thing is that if you run on the treadmill you will develop discipline that will help you when you start to run more outdoors. Believe me, when the weather gets warmer you will want to run outdoors, especially all over Central Park!