The first day is by far the hardest day of the walk, it is up, then up some more, round a corner and surprise, up again, it is a long hard day but remember, once over the Pyrenees mountains it is much easier.
Also remember the weather on the Pyrenees is much colder, we had lots of snow and ice to cope with in early April.
I never really trained for the walk when i did it for the first time and i found my fitness increase over the first few days.
I admire you walking the camino at your age, it is a great experience, one i am looking forward to doing again.