I had just retired from the military and run marathons, including one that is done in a uniform with a military pack, so it didn't seem like a big problem...and I had already walked from SJPdP to Irache before I left to start the Norte so had a 'warm up'...but everyone is different. I also took rest days in several towns and had the perfect shoes and pack for me (probably the most important thing for anyone doing any camino as you know). Since I was walking in the spring, it seemed to me that of the few pilgrims I did see, half were below 30 and half above, at least one I met was in his 70s, and many more in 50's. So if we older folk can manage...but yes, there is a lot of up and down. I can recall thinking 'oh, come on, another one?'...I would take a picture of the mist shrouded foothills and snow capped mountains in the distance each morning, and email it to friends saying 'pretty in the morning light, until you realize you're going to go over them....again.'
there's also a fair amount of road walking, which I know because I can see it in the guide, but I don't recall it as prominent probably because the woods and beaches took my breath away. The only times I recall the road was when it had a narrow shoulder on a winding mountain, like some of the end approach to grandas, and you hope the cars see you. but...make sure your footwear feels good on pavement as well as dirt--the price we pay for outstanding food, and all the natural beauty we walk through when not on pavement.
I'm curious about your month timeline for that entire trip though...on the 17th day of walking I reached oviedo and in ten more reached SdC and in three more Finisterre --that would take up all of August without the trip to Fatima, and does not count the rest days I took in Bilbao, Oviedo, Lugo, and SdC, and the half days in a few others). And I was usually leaving most other walkers behind with my pace (not fast
, just longer periods of walking). I don't know how many more days are needed to reach Fatima but that is looking like pretty long walking days, even if you never take a day off to see the sights (and I would recommend it in those cities, and half days in a couple more---there is soooo much beauty and history on the route you've chosen).