The most miles I bike at a time is around 18.5 miles. I feel like temperatures in the 80's are okay, I break a sweat but don't feel overwhelmed by the heat (and mentally I feel like the 80's are so close to 70, and 70 can feel quite cool, especially if you are just sitting in an air-conditioned room)...At 90 there is some discomfort that takes a bit of joy out of the ride...At 95+ is where I especially try to "not push it", pace myself and stay hydrated. In Southwest Virginia we don't get very many 100+ days (but we do get plenty of sunshine and humidity in the summer).