Just keep working with it, is my opinions. If the soreness is limited to the sit bones, it's probably set up about right. A different one might work better for you, but I would say to stay with what you've got at least for now. You'll know more about what you want later. I seem to recall similar saddle problems when I first got back to riding.

Stamina will come back fairly quickly. Don't overdo it to the point of real pain. Give yourself a rest day, from time to time, but don't quit. Some of the best rides happen when you head out in spite of just not feeling like it.