This advice is from a self-booking perspective, with the idea of booking before you leave or having enough info to book or modify while you are out on the camino:
Start with a list of possible stages. Go back and look at the tour company you first considered and make a list of stages they would have put you on. Then look at any other tour companies and their stages. Many of these companies cater to seniors and also want their customers to book more nights rather than less nights. This means they are likely not having their customers overdo their daily walks.
Guidebooks will often be more helpful in describing the actual terrain of each day's walk. Same distance between towns doesn't always mean same level of difficulty.
Gather several stage lists from various sources and see how they match up.
Then consider their reported distances between stages and generally decide (meaning not commit to a fixed decision) if each specific stage will be too much for you.
Then go look at J Willhaus
above suggestion about gronze and see the different towns and possible stopping points and start making a list of possible stages which fit you and begin to formulate a stage design which suits you.
Once you start formulating your Possible Stage List, make sure you know the distances between each possible stage. JW's example above is great for this. Use that and write each down in a list in a notebook so you can easily combine distances (ex: Sarria to Vilei 3.6 + Vilei to Mosteiro 0.6 = 4.2). You don't have to list these in your notebook for every single town, just for the ones you are considering stopping in, but you WILL take all the distance totals for the towns you are passing through and put those distance totals on your list. (Ex: Gronze lists 18 towns between Sarria and Portomarin. You don't need all of them listed but you must know the distance between Sarria and Portomarin and the distance between Sarria and any town before Portomarin you are considering stopping in).
As you start making a list of possible stages which suit your ability and fitness level ("I know I can make it this far" or "I'm not sure if I can do that distance" or "I know I can't go that far") you will begin to formulate a more coherent list of possible stages.
(As you continue walking in training you might change your mind as you assess your capabilities and adjust possible stages accordingly, i.e. "I can walk further than I thought I could" or "I'm worried about my knee acting up again. I'd better take it easier than I previously planned")
So you take all that info and start making a stage list for YOU on THIS trip. You will have definite stops and possible stops.
The Definite Stops are ones which suit your preferences. Those preferences might include: Ability to book ahead of time; Suitable accommodation; Places to eat; Distance between towns demand you stay there.
Then you take your Possible Stops and either solidify or eliminate based on your needs and criteria: Distance between definite stops; Places to stay, if any; Types of accommodation; Ability to book ahead of time, etc.
One strategy to consider is to book all Definite Stops ahead of time and booking the ones in-between as you are on the road.
If you use Booking(dot)com a great tip is to look at the guest reviews and do a search within the reviews on each specific property you are considering using the word 'camino'.
You will find that people on the camino love using the word 'camino' in their reviews and they are very helpful. Your search results will include results such as "right on the camino path" or "nice place but too far from the camino for us" or "perfect indulgence on the camino after many nights in albergues". The more results in reviews of the words 'camino' and 'pilgrim' the better. Using those words will net you info on distance of hotel from actual camino route, comfort of beds, bathrooms, showers, bathtubs and other things. If you don't see the word 'camino' in the reviews there is a good chance not too many pilgrims stayed there-- and the two main reasons pilgrims won't stay in a specific hotel are 1- too expensive 2- not close to the camino.