Working out

I'm a huge fitness freak. I even ran my first half marathon a few weeks ago and didn't know I was pregnant! Since finding out I'm pregnant I have been so nervous to workout. I decided I'm not going to run outside anymore due to heat. The treadmill however makes me feel like I'm pounding too much. I love the stairmaster and usually do it on level 14 for an hour. I'll obviously discuss this with my doctor, but do you think it's okay to continue? Anyone else working out pregnant?