I've done a century with temp at 105F and heat index of about 112F, and I was really suffering at end, but OK. During the last 30 miles, my feet were absolutely on fire. I also remember a much short ride in similar conditions and called my wife to alert her that I may need a pickup. Just depends on the individual, level of fitness and level of hydration (both pre-ride and maintenance during ride). Balanced electrolytes are your friend.