Exercise & Pregnancy

This is my first pregnancy - I’m six weeks along so I’ve not been to see my OB yet. I live a very active lifestyle - I teach an hour long spin class up to 2x/week and was in the process of training for a half marathon when I found out I was pregnant. I’ve already noticed distance running becomes difficult sooner than usual, but I don’t know exactly when I’m pushing myself too hard now. Any recommendations for where to draw the line in regards to exercise? I plan to discuss this concern with my doctor in a few weeks, but was looking for some immediate advice.