Hardest workout? Or hardest workout ever?

So on Wednesday afternoon when I got home from work I did an 8 mile tempo run with a half mile warmup and cool down factored into the distance so 7 miles at my goal pace which was 15-30 seconds faster then my goal marathon pace. I wanted to do the first 2 miles at 30 seconds faster and the next two about marathon pace and then the next two at 15 seconds faster then marathon pace, then one mile at my marathon pace. I was able to do the entire run at the goal paces (treadmill speeds tend to be different for some reason) and felt solid throughout the run. You can see the data on from Garmin Connect below.

As far as a workout goes in this 20 week program its was in the top 5 for difficulty but not excessively hard or over the top. Yet two days later my legs are still feeling it! So much so that I did not run the 4 miler I had scheduled for Thursday. This is the first significant soreness I have had during this whole program. Not sure exactly what it means, but I am taking it seriously and my two runs this weekend are going to be at a relaxed pace for sure. My resting heart rate is not changing (I use a BASIS watch to monitor my heart rate and it gives me my waking heart rate every day) in fact it trending downwards.

Coincidentally or not I have also lost 2 pounds this week, from 221 to 219, I tend to hover at a weight for a few weeks then drop 2-3 and then hover again. sort of like stair steps. So that's a positive. But I would guess dropping weight would not lead to soreness in my legs, unless the 2 pounds I loss were cushioning my legs.