I see your concern. The whole of Europe is suffering or enjoying a long heatwave - Actually, it is global I think.
Pack super-light, the less weight you carry the easier it will be. You will find that Camino walking is not that many hours each day really so were you to start in the cool of just after dawn you could end your day by noon. The main heat time is 1pm until about 4pm - but then, this is what a siesta is for!!
The forecast is mid to high 30's C. I walked on the Meseta last year in the same temperature and it was fine - just that 1pm until 4 or 5pm that was baking. Mind you, I used a long handled umbrella - you won't believe the difference it makes as a sun parasol! (Long handled or your arm will be up in the air all the time!).
View attachment 45107
I say stay with your original plan and just take it easy. All will be well. (but do pack super-light).
Buen Camino!