Making Gains - No Clue Why

I've had my bouts with a cold, and possibly even a mild case of the flu since Jan 1.

But since then I've seen increases in my stamina and general energy levels that are completely unexplainable.

Case in point was this past Saturday's long run (10 miles). For most of the first 6 miles I nearly forgot I was running. And when I hit the 5 mile mark I altered and extended my course because I felt so good. I wanted to go more. I ended up stopping at 11-12 miles for fear of an overuse injury. Then I hopped on the bike trainer yesterday and mashed out a very hard 90 minutes and left a big pool of sweat under the bike. I was exhausted, but in the good way. I did not ache. I was not looking at my watch saying "oh god if I can only hold this for another 5 minutes"

This is all well and good, and this is not a brag. The frustrating thing is that I have no explanation for it.

No change in medications. I've been training various ways for years. If anything, I am not consistently pushing myself like I used to years ago. No mandatory tempo runs or rides, no alarms if my pace drops below a certain number. Over the past 6 months, at least, my approach has changed. I was not seeing gains so I got into a habit of going harder on the days I felt like going hard, and happy putting along on the days when I did not.

Another remote possibility is that with it getting lighter out earlier, I am a bit more motivated to get out there.

Anyone else have similar observations?