Dire Straits sang that back in 1991 (from the song The Bug, on the On Every Street CD).
Yep, some days you wear the cape, some days it wraps around you and becomes a burden. Some days You're the hero, some days the damsel in distress.
Some days you've got it, some days it's got you.
Most of my training lately has been my "daily" lunchtime rides from work. An hour plus, varying in the 19-24 mile neighborhood (depending on whether there are meetings scheduled around lunch -- WHY do they do that?). They've been going well, average speed slowly climbing. I've had a few that have hit north of 20mph, including all the slow-downs and speed ups from intersections that maddeningly drop the average speed. And that's on the road bike. I've had the tri bike out a few times, and since I don't have a computer on that one, I fly blind until I'm done and can take a look at Strava. No number in front of my eyes egging me on to push harder.
Those days I come back to my desk tired but feeling good. That cape is flapping in the wind behind me.
Today, not so much. Tired, mentally dull, cloudy weather and cool temperatures (that I would have seemed so warm just a few months ago), and I'm just not into it this go-around. It happens. On those days it's often better to just chill, take the day off. Rule #5 doesn't apply here.
There are times when I tell myself that I'll just start the workout, and re-evaluate in 15 minutes. Usually by then I'm into it and feeling better, and can roll through with no problem. But there are the rare times when I know it won't go well, so it's just better to hang it up for the day, recharge the batteries, and take after it again the following day.
If I were going by some real training plan, I'd likely have rest days scheduled in...