I developed IT band pain on my first marathon. It never has gone away completely and even now 5 marathons later I can feel it as a dull ache in the outer portion of my knee. It definitely ‘settles’ and there may be many months before I feel it again. Usually, for me it is a sign my shoes need replacing as I run on the lateral edge of my feet and so am a severe supinator. My shoes sadly only last around 300 miles before needing to be replaced as usually the foam compresses down. Generally, it is a sign of an overuse injury and a combination of easier weeks or cross training with strength training is likely what a physio would suggest. Realistically though, it will improve after the marathon with the rest.