So, while marathons are not much fun in the summer in the US, there are a couple of big time ultra events during the summer heat. Western States is one, but the other that comes to mind is Badwater. This 135 mile race is pretty much insane from the very start, which is in Death Valley, in July. The original idea for the race (first run in 1977 and officially a race as of 1987) is from the lowest point in the US (282 feet below sea level) to the highest, the peak of Mt. Whitney (14,505 feet). That has since been cut short when the National Park Service required permits and forbade competitive events on the peak (read more at Wikipedia).
The finish is on Mount Whitney about 8,000 feet up from the start at 282 feet below sea level, with an total elevation of about 19,000 feet that must be run. Temperatures are frequently over 120 degrees F. Like the coverage of Western, Badwater has provided a fair amount of web content to give you a taste of what the race is like, a handful of videos and a ton of pictures. The field of 94 who toed the line included 12 DNFs and the winner completed the race in just under 24 hours (Oswaldo Lopez in 23:41:40). No desire to try out for this race anytime soon, Florida is plenty hot enough for me.