1- I feel the same. As others have mentioned, our aerobic base is probably not good enough. Hence, shorten M pace runs or slow down a bit.
2 - of course, JD states many times in his book that these times are just suggestions. nevertheless, if you (like me) have never been running in a club/college/HS/whatever, the first time you do an actual workout as opposed to just jogging you'll almost have to follow these paces, or try to. in fact, it's quite incredible how well this works for many of us. then, after doing a couple of workouts, you'll see where your strengths and weaknesses are and you can adjust. once you're experienced enough, you don't need the tables anymore, sure.