I had a b ad case of ITBS related to cycling a few years ago, but since the Henley 1/2 M I've had an on/off pain on the outside of my knee that doesn't feel the same at all. It's most noticeable going down stairs and seemed to wear off, only to come back after an easy pace 8 mile a week after Henley.
Henley has some cruelly steep downhill sections and I'm convinced I've got a lateral problem after some over fast corners on a camber. I always do ITBS stretches since the cycling injury kept me out for 3 months, so I don't think it's ITBS.
Having unsuccessfully tried to run through it, I'm going to rest and talk to my osteopath on my next visit, but lateral ligaments sounds like a promising diagnosis John.
Maybe follow RICE and HARM (avoiding heat, alcohol, running and massage) in the meantime, with the exception of massage on the quads to help in case it is ITBS.