New shoes, help?

When I get new shoes I always break them in gradually. I rotate them with the old shoes for a week or two and build up milage on the new shoes. I start wearing the new ones exclusively after about 2 weeks.

I bought a new pair a few weeks ago and my first run in them was a 2-hour ~12mi run. No blisters, no issues. Ran a 12k race in them yesterday, and they felt great.

You say it was the same model, but was it the exact same, or did they update the models? If it is the exact same, its possible that pair has been sitting on a shelf for 6 months or more, and may be stiffer than a brand-new pair. If they changed the shoe slightly, that may account for it.

You should be able to take them back, but you may only get store credit or have to pay a restocking fee - depends on the store. You should also be able to test them in the store (the place I go has a treadmill for testing new shoes) and the staff should be able to help pinpoint the problem with the shoes, and recommend another make or model.

I run in Mizuno Wave Riders and they are ready to go out of the box.  Maybe it shows just how bad you old pair were.