Don’t worry about breast milk. Most of the mummies only have their supply kicked in at day 5.
And please do not massage your breast now because it will cause preterm labour.
As for exercising, you can brisk walk around your house. Many mummies including myself are active throughout the entire pregnancy, but eventually had long labour.
So it might be a myth as well.
Nonetheless, don’t worry too much!
Breathe in and out!