Hey your single speed Schwinn can give you one heck of a workout. If that is where your comfort zone is, stick with it. Maybe you just need to change the gearing a little?
As far as your TREK goes, do you know if it is the right fit? Have you considered going to your LBS and have them check you out on it? It will feel different than the Schwinn no matter what but starting with the right fit for your body will help with your overall comfort.