Notice his attitude he'll soon tell you when he's had enough. Don't argue with Mother Nature. Check for structural soundness, and take any indication of filling or heat as a sign that you are pushing too hard. Do not wait for actual lameness to occur. But your greatest danger during conditioning or competition is dehydration and the consequent electrolyte imbalance.
Work produces heat as a byproduct.
Horses dissipate heat by shunting blood to peripheral blood vessels, there for body heat to be radiated off. They also pant like a dog, literally exhaling hot air and breathing cooler air. But their most efficient means of dissipating heat is evaporation due to sweating. The skin looses elasticity, becoming more like parchment, gut motility slows down, and the blood actually gets thicker. Your veterinarian can teach you to perform simple tests to check for dehydration by monitoring skin response, capillary refill time, gut sounds, and quality of mucous membranes.