I'm 67 and take trips even longer than what you are planning, but I try to avoid 1- and 2-night stays, and I make a real effort to find enough to do at each stop to justify spending 4 nights there. I feel like you are planning to move around a bit too much. For example, flying to the Canary Islands for just 3 full days (with a ferry trip on one of those days) isn't something I would do. Although I would like to visit the Canary Islands at some point, I have so far twice opted for the Portuguese island of Madeira instead. It is drop-dead gorgeous and worth a 5-night stay if you want to spend some time walking the paths along the levadas. Those are flat, but you would need transportation to get to and from the levadas.
You have just two nights in Porto, so really just one full day; I think an extra day would be worthwhile so you could do something outside town, such as a trip to the Douro Valley.
With three nights in Santiago de Compostela (not super-easy to reach from Porto), you have only two full days, yet you are planning two day-trips. The historic area in SdeC is quite large; when will you see it? I haven't been to Ourense, but I do like A Coruna.
Oviedo is more historic (with some extremely old churches--not walkable from the center). Gijon is livelier. Gijon has a small historic area and some attractive late-19th/early-20th century architecture.
The Picos de Europa scenery is great, and the old stone town of Potes is charming (though touristy). In 2016 there were once-a week bus tours (I think on Wed.) from Santander, but they ran only from early July to early September. There may be public-bus transportation between Santander and Potes. Googling may turn up something. Otherwise, with four of you a private driver should be affordable.
I found Santander itself imminently skippable. It suffered a devastating fire in the 1940s and has very little historic architecture, though the beach is magnificent.
The Basque Country is extremely scenic. If you have time you might enjoy the slow train trip between San Sebastian and Bilbao; the bus is faster. I prefer the much less touristy Bilbao to San Sebastian, whose entire historic area seems to have been turned into tapas bars. Get an early start on the day you head to Bilbao or consider spending a few nights there. Vitoria-Gasteiz is also very interesting. Note that the historic district is on a hill. There are exterior escalators to assist you in reaching the top, but it could be a bit difficult for the mobility-challenged. Other highlights in that area for me were Hondarribia (again, there's an uphill walk to the oldest part of town) and the small coastal towns of Bermeo (near Bilbao) and Zarautz/Getaria/Zumaia (near San Sebastian).
Pamplona is only OK; I thought it offered less of interest than most of the other places I visited in Spain.
Zaragoza is worthwhile, but it can be extremely hot during the summer. I recommend staying near the old town rather than out near the train/bus station, though you might get a good deal on rooms at the EuroStars hotel there. Unfortunately, the area is a food desert, and you are a 15-minute walk from the Moorish palace and about a 30-minute walk from the historic center. There are buses and taxis from the station, but it's annoying not to be able to walk out of the hotel and find food within a few blocks. The hotel restaurant closes during the worst weather and offers a limited room-service menu; avoid the pizza.
I think Mallorca is worth more than 3 full days, especially if you will be traveling around by bus.
If you need more time elsewhere, I'd consider skipping Valencia (you have only one day there) and especially Alicante.
Avila is a wall and not much else; skip it. Salamanca is very nice but a very long day-trip from Madrid. Toledo is closer and I prefer it; I spent multiple nights there. Cuenca is nice; buy that AVE ticket early to save a lot of money.