Good Days vs Bad Days

I try to keep a fairly good log, and I do have a training plan. The intention is to find the right workouts that will keep me in shape, allow me to make some improvements, and most importantly keep it injury free and fun.

I guess you can say every workout is a good workout. But each year I become more and more convinced that I will have good days when I feel like I can run or bike forever, and bad days when I want to stop running after 20 minutes.

But the reasons for the good days vs bad appear very complex and almost impossible to predict.

A bad day was a run this past Saturday. I started out fine, but the heat/humidity was off the charts where I live. I was ok for 3 miles, but after 4 miles I was sweating so much my shoes were making that squishy sound. I stopped briefly when I could not keep my eyes open from the sweat. I literally looked like I jumped in the pool. I wanted to do 8 or 10, but only did 5. So I did 4 repeat sprints of 200M before calling it a day. I was completely spent.

Then there was Sunday. I took the bike out, but based on the day before I had very limited expectations. But something happened. I did 3 miles of warmup and then spent the next 26 miles riding harder than I had in months. I was going almost 1-2mph faster (both into and with wind) than I had on even some races. I had done this bike course so many times, I remembered all too well many past rides where I faded after point A or Point B. Not today. I rode hard the whole way, even picking up the pace that last few miles. I could have gone longer but had to be back home for family obligations.

When I got off the bike I was drenched, but it was a different feeling, a good feeling. Later that day I did 60 minutes of hard swimming in the town pool.

So what does it all mean? I wish I knew.

My heart rate was almost 10 bpm higher while riding the bike than during the run the day before. Its usually the opposite.

Something I ate, or didn't eat? cant say

Better night of sleep before the bike? not sure either

Is there a way to build off that good workout on the bike? Or does an intense ride like that suggest a day of rest?

For those who have those swings in good days vs bad days, are you still searching for reasons yourself, or by now do you just accept it?