I do 5-10k runs once or twice a week. While my legs inevitably get tired, last time they felt unusually heavy before even hitting one kilometre. Especially the ankles which felt solid rock.
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.