I feel pretty good today after my two-hour run yesterday

I've mentioned in a previous post that I felt tired after my last two Monday runs that incorporated cadence drills. I assumed my being tired was due to the cadence drills that caused me to run 30-second spurts at a slightly faster pace. I didn't do drills in my two-hour run yesterday, and I feel pretty good today. This may be a confirmation that the drills were responsible for my being tired. However, for two Saturdays I ran fartleks during a one-hour run, and the fartleks were faster than my cadence drills. I felt fine after the fartleks. So, I'm not sure why I feel good today, just that I do.