Still Sluggish

Monday I did 5 with the boys from work. Felt awful. Yesterday I did a short 4.2 - first mile easy, 2 mile pickup, one mile cool down. I did the 2 mile pickup practicing race pace - about 7:30-7:40 range. The good news is that I was able to hold it without much pacing help. Can I keep it up for 13.1? That's the question.