Every person is different and has different tolerance and physical ability to tolerate extreme heat and sunshine. IN GENERAL:
- Hydrate, even forcing yourself to consume water regularly throughout the day. Personally, I consume .5 liters of water or electrolyte solution every hour, whether or not I feel I need it. In temperatures over 30 degrees (c) make that one-liter per hour.
- If you stop having to pass urine, be careful, this is an early sign of heat exhaustion. Your body starts to send water to your vital organs to try to cool the body.
- If you stop perspiring / sweating, this is also a sign of heat exhaustion.
- Light-headedness and dizziness is a final sign of heat-exhaustion, just before you pass out. STOP IMMEDIATELY. Ask others to help you. Seek shade, cool-down, hydrate!
- Start early, finish early for the day - well before noon.
The European weather maps for northern Spain, for today 26 June, show that Burgos and to the east is frighteningly hot, whereas Leon and points to the west are relatively cooler, but still hot. Thus, if you are attempting the Mesta from Fromista west, anytime soon:
- Take double or triple the water you normally carry. Share it with others who need it.
- Wear a broad-brimmed sun hat with appropriate ventilation. You need a hat with 360 degree, all-around, protection. A ball-cap is not enough, IMHO. The hat should be tan or light colored, NOT BLACK or another dark color that concentrates the sun's heat.
- Obtain and use a light-colored umbrella to create your own shade. Stick it in your sternum strap to hold it in place while using hiking poles.
- Wear a wet Buff, or similar microfiber tube accessory garment, head cover to aid evaporative cooling. A wet Buff can also be worn around your neck or wrists. Both are pressure points. Placing evaporative cooling there will aid in cooling your entire body. Absent having a buff, any microfiber towel will work nearly as well in some positions, like head cover.
If you find shade along the way, USE IT. Take a break. Take as many breaks as you feel you need.
When you stop, consume water. Try to lie down or otherwise elevate your feet and legs to allow / improve circulation to your brain and torso.
Listen to your body.
Take care of your feet. Change sweaty socks during the day to avoid friction and blisters. Pin the sweaty, wet socks to your rucksack to air dry.
Hope this helps. Vaya con Dios!