How do you train safely in Indian summer heat?

Shift the time first — early morning or after sunset avoids most of the risk, and no amount of toughness compensates for training at 2pm in May. Hydrate before rather than only during, and for sessions over an hour or heavy sweating, replace salt too, since plain water alone can leave you worse off. Reduce intensity for the first week or two of a heat wave; acclimatisation is real and takes about ten days. Stop for dizziness, nausea, headache, or if you stop sweating — that last one is an emergency, not a sign of toughness.