A place for runners.
Last time this happened to me it was because I didn’t eat enough beforehand.
Usually I eat a decent lunch then go for my run a couple hours later - maybe eating a banana 15-30 min before. That day because there were thunderstorms forecasted I skipped lunch and just had a banana and a cliff bar before my run. Got tired pretty early on that day.
Perhaps the body builds certain habits. I go for a run in the early morning on an empty stomach. Always been the case.
Maybe you needed a rest?
Do you do any kind of warmup?
I don't remember! Sometimes when I don't feel lazy I stretch beforehand (I should be more consistent). But I usually begin by simply walking then elevate the intensity progressively.
My biggest suspicion was breathing. I somewhat remember that I wasn't properly inhaling and exhaling, sometimes would hold my breath unconsciously.
This morning I made sure to stretch and actively breathe. Didn't face an issue.
Have you had covid recently?
...or simply a cold or flu?
None whatsoever. I went on a run this morning and didn't face any issue.
I took up running some years back. I went from my body hurting repeatedly to being able to complete two miles without stopping in two weeks time. It's amazing how quickly the body builds up tolerance.