For long efforts in warmer conditions, hydration is obviously necessary but easy to concentrate on that when keeping one's body core cool is proportionately more important. In such conditions, people are more likely to go down with heat injury than because of dehydration. The cooling effect of cycling speed means this is more markedly the case for runners.
I offer this anecdote (which has influenced my opinion on this). The Three Peaks Yacht race (starts 15 June this year, from Barmouth (TINAT note)) takes in Snowdon (normally at night), Scafell Pike (often by day) and then a day plus later, Ben Nevis. The Scafell Pike leg is/was the longest: from the coast at Ravenglass (port changed nowadays) to Wasdale Head (20km); up and down (10km + 900m); back to boat. Three plus hours done in the middle of a hot June day, well hydrated, heading back with 10 miles to go at a brisk but steady pace (90 minute half marathon pace (with rucsacs)) my heart rate started to rise markedly (to above 180bpm), with no associated increase in pace and no self-perceived increase in effort/exertion. So the next time we reached our support, water went not down my throat but over my hat/head, shoulders down my back and front and on my legs. My HR dropped immediately back to 'normal' 160bpm (say). There is an inclination not to 'waste' water: it's needed for drinking. It's needed for body cooling too.
So my advice is (as @Hellymedic says): regular 'external' cooling and don't be concerned, any more than normal, with additional hydration: drink to thirst. This means keeping at least one bottle for plain water, whatever potions/tablets/powders you care to add to the other.
In a previous 'life' the Institute of Naval Medicine led some careful research into what liquids were best for maintaining proper hydration over a hard endeavour of 8 hours (across Dartmoor), in addition to eating ordinary food, a banana or 5 and a pasty (salted). The conclusion was: . . . . . water.
Phil W said "Rehydration with water or a sports drink is the cornerstone of treatment for heat exhaustion."
I can find where you've quoted this from but am surprised at this. The 'cornerstone' of treatment (indeed ideally for prevention) for heat exhaustion must be body cooling.
To quote another site (on treatment):
"Anyone who suspects that they have heat exhaustion should immediately take steps to cool down. These can include:
moving to a shady location
removing one or more articles of clothing
resting out of the sun
turning on a fan or the air conditioning
running cool water over the skin or applying cool, wet towels to the body
drinking fluids such as water and sports drinks"
Shade, strip, fan, soak, and only then, drink.
If you have pint of water, most of it needs to go over the patient/victim, not down their throat.