I had the same problem. I had been cruising through the program up until W5D3. I felt like I was going to die after the first 12 min at a snails pace. I had to walk a few minutes then struggled through the remaing 5. Thought about repeating the run but I've decided to put it behind me & move on to W6D1 (which should be a cake walk in comparison!)
By the way, I'm 43, at my ideal wt and exercise regularly.