You take where you are and move forward--not where you want to be. If doing 20-25 minutes is doable for you, than most certainly that is fine. I often remind people that while doing too little will not bring results, so will doing too much. That said, exercise for the long term is what you see yourself maintaining not just when losing weight, but days and weeks beyond that time. This is why it is best to do what you see you can maintain for a lifetime.