Until 15 days ago (when we returned to working from home) I was having a good run of commuting time on the bike. Minimal intrusive weather events and lots of days pushing the pace meant my time to work was consistently shorter than what I was used to. I felt like my fitness was progressing. But commuting "work" and training "work" are usually quite different. In duration and impact.
Still, it is regular riding of a decent length and I've proven I'm pretty good at using commute time as training.
However, I've felt pretty slow on the bike - especially the mountain bike - when I'm not commuting.
Given the slightly muddy conditions of late, most of my MTB time has been on the single speed. I got a single speed years ago in part because there are no derailleur gears or expensive drivetrain bits to wear out. I am on my third single speed now because they do run forever with little maintenance and they're a lot of fun.
Last weekend I had two decent rides on that bike. Both felt like I was struggling for speed.
Enter Strava. I got several PBs on both rides. Mostly climbing ones (where fitness matters most). Including one that was set in a race about 8 years ago. While Strava is GPS reliant and therefore not accurate enough for proper race timing, to-the-second results are fine and I don't care about a second here or there anyway. On a 3 minute section, being one second out matters in a race but not in a "how fast did I go?" query.

The coaching lesson from this is one I've seen many times before. How you feel and how you go are two distinct and separate things. The feel slow, but are fast situation is infinitely preferable to the feel fast, am slow situation at the other end of the spectrum. For me personally, the feel fast, am fast situation leads to tactical errors.
If only there was some good racing to really test my fitness on...