Never dealt with a dog exhibiting heat exhaustion however every fall if the pheasant season opener is a warmer than usual day....I read about many upland hunting dogs which get devastated by heat exhaustion....some die.....same thing every year if the temps are above normal in October for opener.
They give the same common sense advice every year this happens and it essentially is exactly what you did....watering the dog regularly, taking some rests and looking for classic symptoms...excessive salivation,vomiting, tongue and other areas in the mouth take a brighter than normal red appearance. If I recall properly, if the gums, lips etc start to turn a grayish color...you have real problems as the dog is nearing a critical point where action must be taken.
As far as a scuffed/abraded/cut pad from an 8K / 5 mile walk perhaps it's more of a cut as you describe it rather than his pads being worn from a distance of that length. Seems odd that an 8K would wear a dog's pad down to a bloodied state from that distance....assuming the surface the dog was walking on wasn't ridiculously abrasive. maybe Charlie simply just cut the rear pad on a sharp object along the way.
I have no idea how old Charlie is but I have heard many in here profess if it's a younger pup, long distance walk/runs might be something to avoid when they are still sprouting. And of course an natural/earthen surface is superior to a hard concrete/asphalt surface for long treks or runs. In the summer the asphalt can be a real problem when the sun has it heated up considerably...another common sense idea some folks might forget about about regarding their dog's pads.
Sounds like Charlie is fine and did have a blast....they're tough and no little cut in their paw is going to get in their way of being a dog...but always worth keeping an eye on and making sure it heals properly.