red spots & itching after long walks

I am training to walk in a marathon, and am now walking 16-20 mile distances.
After a recent 16 mile walk, my feet (most severely near the ankle but spreading over most of the foot)beagn to itch uncontrollably.
I noticed red spots on my feet when the itching began, about an hour after I finished walking.
Nothing will stop the itch- I’ve tried icing the affected ares, soaking the feet, Cortizone ointment and lotion.
The red spots and itching remain several hours after the walks.
I never had any problems during shorter walks, and my feet don’t sweat overly much.
Any suggestions would be most appreciated, since I still have a lot of walking left to do 🙂