My plan is to take 19 days to bike from Pamplona to Santiago, with 6 flex/rest/weather/tourist days. Prior to my start I am going to give myself 3 days to get over jet-lag split between Madrid and Pamplona. The reason that I have 6 flex days in my schedule is because on average about 1/3 of the days in May have measurable precipitation. Some of those days could be horrible storms and others could be light rain for just a few minutes. In some of my training rides, I have trained in rain, just to be on the safe side as far as gear and conditioning. Spending time in churches, taking in museums will also be an important part of my Camino for me.
In the past in training for both long bike rides and half marathons, I find that my body responds best if I have at least one day off a week and one day of very light exercise. So if I add tourism, then 2 to 3 "rest/recovery" days per week are what allow me to keep going week after week after week without over-use injuries.
To do a combination bike and hike, I would think you would have some extra gear that you won't need for the other transportation mode. Things like bike helmet, bike shoes, padded bike pants, padded bike gloves, etc. Other things like hiking boots, treking poles, large backpack would just be extra baggage for the biking part. Also the muscle groups you will be using will be different for each mode of transportation. Then again, if you have trained for this, it shouldn't be a problem.
I am curious why you are not biking farther like Astorga or Ponferrada? Is it that you want to minimize your biking so you can maximize your hiking time into your schedule? Again, I would urge you to plan a few flex days into your schedule, if for nothing else some bad weather.
Good luck.