I don't know if they are likely to hear this from others or not, but it is the first piece of advice I give to all first-timers. Give yourself more time than you think you will possibly need. You don't know in advance what you will find your best daily walking distance to be. You don't know in advance if issues like tendonitis might lay you up for a few days. The last thing you want is to find yourself racing to Santiago to make a plane. Give yourself that cushion. If you need it, it is there. If you don't need it, you can spend an extra few days in Santiago, you can walk to Finisterre/Muxia, or you can see some of the other wonderful things there are to see in Iberia.
When I went with my son in 2016, I thought it would take 33 or so days. That's what
Brierley and the other guide books indicated. I booked our flights so we had almost two months in Spain. It ended up taking us 37 days to walk to Santiago and another 4 to walk to Finisterre. With the extra time at the end, we did some sightseeing in Spain. We found that my son's blisters got
really terrible if we walked over 25 km day after day after day but remained under control if we generally kept it below that. I shudder to imagine what his feet would have been like on arrival if we had to get there in 33 days.