Andria, it depends on your level of fitness on what a beginning goal would be. What I would do is perhaps pick a number, say 15 minutes then do it for that long. If you can only go to 10, then the next time go for 11. If you can go to 15, then next time go to something like 17.
As far as what is better slow and longer or faster and shorter, I would say do intervals and mix it up. Go at a steady pace that you can keep for a few minutes, then go faster and/or higher incline for a minute, then go back to your original pace. Keep doing that off and on throughout your workout and you will be able to go longer than if you had kept up the faster pace but your heart will get a good workout as well as your body.
You can't out-exercise poor eating habits.