I had ITB band issues after a hilly 200k with a very poor bike fit and not enough training, exact same symptoms as you. I would be fine for the first 8 or 10 miles and then sudden pain. Most likely it's damaged and you should be icing it whenever it hurts.

There's two things you can do to recover (from my experience):

1. Stretch the ITB as much as possible, google "ITB stretches" and you'll find several good ones. Also, if you decide to keep riding, take it easy and stop as soon as it hurts. Ice it when you get home. I did this for an entire summer, progressively increasing my riding distances until it started hurting and then starting from day 1 again. It sucks.

2. Stop riding for 3-4 weeks. If you do this now you can avoid months of new damage. It's extremely hard to do but an accident forced me off the bike (shoulder injury) for 4 weeks and once I tentatively got back to riding my ITB was healed and has been 100% ever since.