Again, Natasha is reading my mind. Consistency is key here, just like with any workout. As you do it more frequently, you will increase your strength and endurance. If you go as long as you can, and do the workout at least once a week, try to add on a few more reps every time. You'll be doing the entire series before you know it. It will still be challenging, but you'll eventually be able to get through it.
I had the same issue, but the more I added it to my rotation, the better I performed.
Eva