Does one need to book a place to stay or one can just walk in?Walk in is very unusual and not recommended, except in case of a business hotel.
I have all the names of the hotels but could someone advice if the planned is feasible for 5 days?
Yes, it is feasible. Quite a lot of driving, but feasible.
Most challenging will be finding your way through the expressway jungle of Tokyo on your drive from Narita to Shiobara. Try to study Tokyo's expressway network in advance.