Are the Mach 7s a lot stiffer than the Mach 6s?

I just got the Mach 7s after using the Mach 6s for a year (500+ miles), and they feel soooooo stiff. I know my mach 6s are super broken in by now and maybe even worn down, but I don’t remember them being half this stiff. I remember them feeling like clouds with springs. I went on 2 3 mile runs on the treadmill with the Mach 7s and they felt hard as a rock, and by the end of the run my left foot was even a little sore. Do I just need to break them in more? It’s probably too late to return them but I’m not sure what I should do.