Tempo...errr...long run

I've been a bit busy with work and studying lately so I'm trying to make my sessions count. So although I haven't done a run longer than 6.5 miles outside of a race since April, I thought it wise to do a 7.3 mile tempo run today. That run DEFINED pain. Holy moly. And the thing that sucked about it is that I felt like I was running through corn syrup. I don't know if it's my form, the extra couple pounds I'm hanging onto, or the fact that I don't like running tempo without a bike ride warm up, but it was just very sluggish feeling. I was even holding to a 180 step per minute turnover. By the last mile I was just totally spent and wanted to throw up. But when I looked at my watch I realized that I did the 7.3 miles in 56 minutes for a 7:40 average. This is really not at all shabby for me! So I ask- why did it feel so slow? I've done much slower runs and felt much stronger. Maybe it's just my inadequate training lately, but I felt like a hippo. I think it's time to watch the diet and run more...