With the approval of your health care provider, you should be able to continue your running until it is no longer comfortable to do so. As a marathon runner I will assume that you are in touch with your body. Listen carefully and heed any internal messages to slow down or moderate your training. As long as you stay well hydrated and avoid running on hot or humid days, your baby, theoretically, should be fine. However, hydration and environmental temperature is very important. Previously, the guidelines given by ACOG were to avoid letting your heart rate increase above 140, not to exercise longer than 40 minutes so as not to let your body core temperature increase for a significant amount of time. Due to more research in the area, it was found that the guidelines can be more broad, and some of the information was incorrect.

If you're already in good condition it may not be necessary to limit your heart rate to 140bpm. Listen to your body. Monday you may have loads of energy and a fantastic run, while on Wednesday your energy may be low and you might feel like a mild jog, or even a walk. Listen to your body's need to rest and recover.

During pregnancy your blood volume increases, which also facilitates the extra blood needed for your working muscles and to maintain oxygen flow to your baby. This increased blood volume also allows for keeping your body temperature at acceptable levels during exercise. This does not mean you can liberally work out in areas lacking ventilation. If you do so, your body has to work even harder to keep you cool, and your baby comfortable. There are so many things that go on with the body during pregnancy that I'm sure that even the current "experts" will have more to tell us next year. So this is why it's imperative to listen to the intelligence that your body and baby has to offer. During pregnancy, your intuition is heightened.

With your doctor's approval, and as long as you feel good and eat the appropriate amount of calories, you should still be able to do your long runs, hills, etc. If you continue long runs, track your calorie intake to make sure that you are not going into a calorie deficit.

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