Hi Nat
When my IVF cycle was confirmed, I slowed down on my marathon training, but still continued to do light jogs and crosstraining and the gym. I maintain my heart rate at 140bp for a normal gym session but when jogging i can go as high as 170 for 1 min and reduce to a power walk and get it down to 129bp, than back up. This still trains your body and is better for burning fat. I do this outdoors as its much cooler now in Melb and so I do not overheat.
The moment I start to feel hot i know that that is enough. I guess the point is to just listen to your body, there is no right or wrong answer, we are all different and all can sustain different amount of exercise.

Good luck.