Boot sizing advice

Hello. I need some help on sizing for boots.

I bought a pair of GORUCK MACV 2 combat boots for running/backpacking. Put them on today, and they feel awesome; however, when I walk, a little heel slip occurs.

I haven’t bought boots in years, and I’m not educated on how they should properly fit. I thought this community would be the perfect place.

My foot feels great—I just don’t want hot spots or blisters when running/walking. Toe box is perfect and everything—just the heel. Not sure if it needs broken in or not.

Thank you for your help!