In my first year of running I got ITBS , I managed to limp through training as it was only a minor niggle and completed my first marathon, BUT I DIDNT STOP RUNNING. then finally it felt like someone stabbed me in the knee and I could barely walk or drive for months. and I had Berlin marathon coming up. I smothered my knee in volterol cream and used an ITB strap, and managed to get round. (after no running at all for 4 months)
Lessons learned were if you are close to the end of training you can probably limp through if its not too painful. but if you don't stop it will cripple you. ITBS is evil and don't ignore it.