Marathon Training: Soreness, Hydration, and Shoe Recommendations

Hello! I’m training for my first marathon, running the Hamptons on 9/28, and I’m signed up for the 365 program. I just ran 15 miles yesterday, my longest run to date (previous was 10). Today I’m pretty sore and have a few questions:

1. After the run, I was covered in salt. Is this normal? I drank a glass of water with electrolytes the night before and had two gels during the run.

2. Should I avoid the 8-mile Saturday run if I’m still sore tomorrow, or do it at a slower pace?

3. I’ve been training in barefoot shoes for the past 2-3 years, but my feet are getting sore with longer distances. I’m considering switching to running shoes. I tried the New Balance Fresh Foam X 1080 V12 last year, but I believe they were a contributing factor to previous knee and hip pain (I had/may still have a baker cyst - MRI in Dec ‘23). Do you have any race + running shoe recommendations that aren’t too high? Any store recommendations in the city for finding the perfect shoe?

Thanks for your help!