Expose yourself to those hotter temperatures to build tolerance. "Take your workout to a hotter environment one or two times per week to better condition yourself overall," says Gahan. "You will then find that you are able to endure more when indoors and conditions are ideal," she says. So, maybe it's doing a warm-up outside before getting into your main workout, taking your runs outdoors twice a week, or running a lap or two around the block in between HIIT circuits.