I've been an athlete all of my life and have never had to pace myself. I use a heart rate monitor when I do higher intensity exercise, like spinning. I do feel my body warming up but try to keep my heart rate under 150. This sounds like a stupid question but, how do I know if my body temp is too warm?
Re: Overheating...
ACOG's recommendation is to never let your core temp get above 102.2 (why hot tubs are on the no list). What that feels like, I haven't really clue.
My heat test came in hot yoga classes, where the room is pumped up to 90-100 degrees. (Duly noted that it's probably not recommended for pregnant women to do hot yoga, but I teach yoga and I wanted to check out this teacher.) Even with the extra heat, extra fluids and just not going so hard seemed to work for me.
FYI, another barometer to monitor your exercise is PRE (perceived rate of exertion), using a 5-8 out of 10, often described as being able to carry on a conversation while you exercise. Although a lot less quantitative, this allows a lot more discretion for the exerciser.
Just found an interesting article on athlete pregnancy. The expert suggests a rectal thermometer every 20 minutes. I must admit I am not inclined to do this myself... just took it to be a sign that it must be really difficult to judge our body temps if that's the best option they can come up with!
https://www.pregnancy.org/question/continuing-vigorous-workout-through-pregnancy