3 1/2 weeks ago broke 4th metatarsal nondisplaced. In walking boot. Yesterday walked a lot & today top of foot is sore. Could I have displaced the fractured bone by wearing the walking boot too long?

Unlikely based on. time already passed since diagnosis and you were wearing the walking boot. It justs sounds like you are appropriately sore due to the time on it. If it does not improve with some elevation and rest overnight see your physician is symptoms are bothersome but unlikely anything else needs to be done.
Aggravated injury. You may have aggravated old injury . Give it some rest , it should get better in few days. It takes 4-6 weeks for non displaced fractures to heal, it takes 10 weeks to feel normal , provided you give rest.