How do I run longer on a treadmill when im unfit?

So im going to the gym and trying to improve my fitness and lose abit of weight. I want to be able to do the 40minute cardio that is recommended but when i run on a treadmill i get tired within the first 5 minutes. This demotivates me because im just not fit enough to do the cardio that is needed. What can i do to make me last longer, or what can i do that will be effective in helping me to get fitter. Please help. Thanks