Worst Run EVER!!

On today's triathlon workout menu was a 50 minute tempo run it was meant to look something like this:
Type: Run
Planned duration: 0:50
Warm up 15 minutes at 10:00+ pace. End with 3X 20 sec strides.
Main set:
1X 7 minutes @PE 5, 1X 6 minutes @PE 6 and 1X 5 minutes @PE7. 2 minutes easy jog in bewteen each set...
Cool down the remainder at PE2...
Make sure HR comes down...
Focus: form, turnover, relaxed upper body.

I knew it was going to be a hard workout but not only was it hard I did not do very well at it. About 30 seconds into the warm up I could tell I was going to have issues. I felt sluggish, heavy, and was having trouble breathing. This was not going to be good. I did it anyway the tempo part of the run was really really hard but I wasn't even going that fast. It was also really hard for me to even bring my heart rate down. I had such a good week last week but after today's run I feel really disappointed.... I guess everyone has a bad day and I have to remind myself of this but it is hard not to be unhappy with these results...