Exercise or not?

You want to make sure you keep your heartrate below 140 beats per minute, drink lots of water and try keep as cool as possible - the risk is if you exercise too hard out you raise your internal temperature and because babies can't regulate their own temperature you can end up 'heating' them too much. I would imagine that this is only in extreme circumstances and for prolonged periods of time though, it sounds like you're doing everything right by monitoring your intensity and walking when it's cooler and it's recommended to keep active during pregnancy and walking is a great way to do that

HTH

When I was in my 2ww I was exercising like normal which for me at that time was pretty intense (circuit training with no rest) I didn't think I was pregnant at the time because it was our first month of trying but I remember one workout feeling absolutely exhausted and wondering why I felt so unfit! We now have a very healthy 8 month old so it obviuosly didn't do him any harm. The key is to not suddenly increase what you normally do, I've heard of marathon runners continuing to run long distances well into their pregnancies because their bodies are so used to it.

A real easy way to monitor your intensity is to make sure you can still hold a conversation while you workout, or at least be able to talk a bit. If you can barely get any words out then you're working too hard.

Good luck for testing!