Have done a fair bit over 500 miles in a day and fairly frequent 300 mile days. The longest runs I've had were mainly motorway blasts across Germany and across France in 30 to 35 degree heat (not Fahrenheit!) and felt a bit like sitting in a very large hair drier for several hours. Have also done just south of Malvern to Aberdeen in January in one hit...that didn't feel remotely like being in a hair drier.
