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.
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.
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.
Maybe you needed a rest?
Have you had covid recently?
…or simply a cold or flu?