Easiest 5-6 day cycle route

Between Burgos and Leon is the flattest. You are on the Meseta the flat Spanish plain.

It is not a 5 day cycle so if you carry on to Astorga that will add another day. Stops could be Castrojeriz, Fromista, Carrion de los Condes, Sahagun and Leon

Mundicamino has elevation maps http://www.mundicamino.com/

Way out of Spain, but still an official pilgrim route, is the Donau Radweg (Danube Bike Route) in Germany. The trail follows the Danube from one old town to the next, on a car free path. It features the yellow clam shell markers we all love. and has amenities such as inner tube vending machines and mini locking garages for your bike and gear so you can stroll around town and not worry about your stuff. It is very flat, and you'll see many older riders.