Colleen,
Listen to your body. A mile is awesome for just beginning, especially since it is hilly. If your goal is to become healthier, just keep riding and pace yourself. As you stated it is not a race. See if you can go a tad further each day or every other day. As soon as your body becomes accustom to the bike you will notice it will become easier and more enjoyable and you will find yourself going much farther than you ever imagined. Exercise can be fun.