For running, this sounds very much like:

https://en.wikipedia.org/wiki/Iliotibial_band_syndrome

Non-specific knee ache leaving you crippled walking down stairs, and some other specific movements that cause the iliotibial band (on the outside of your knee) to rub over the bone/bursa. Super common running injury.

For me, the fix is - ice, stretching, instantly stop running if I feel a twinge, and, most importantly, ramp up mileage very very slowly (first week, 1 block, second week, 2 blocks, third week, ruin 3 blocks). Learning what it really means to "ramp up slowly" was the big challenge for me - it's always too tempting to go "hey, I used to be able to run 10mi easily, I'll just crank out an easy 5" - boom, limping for a week.