Have you ever done a workout for the first time and nearly died doing it? That is what happens to me every time I try something new. I tried a Bob Harper Beginner workout and could not finish it (Just a little embarrassing). However, I tried it again the next day and got all the way through the workout.
You have to remember, when you start a work out, it is not going to be easy. The phrase 'practice makes perfect' applies full heartily in this situation. When you try something new, make sure you don't just try it once, say that it is too hard and never do it again. It takes time to get your body use to the workout. Try the work out again, you will be surprised at what your body can do.