The first time I did Phase 1, I had the same thing. I usually work out 5-6 times per week. During that time, I usually do about 40 minutes of cardio and about 25 of weights. Cardio killed me! In the past, I did the EFX/ Elliptical at levels 10 and 8, but during Phase 1, I was lucky to crank out 6 and 4!!! I was actually advised by my doctor to cut back on exercise during those first two weeks.
The good news is that my energy returned with a vegeance!! Because I exercise regularly, one of the first things I introduced back into my diet was fruit. My body didn't like that so much, so I started adding in a little bit of bread (whole grain wraps, rye bread). That did the trick for me!
One of my biggest motivators for coming back to Phase 1 is knowing how good Phase 2 makes me feel. I hope you find the same