It was based on Ernst's own diaries. There are accounts of these runs also in contemporary newspapers. However, these are often vague second hand accounts. In those days there were no such people as foreign correspondents, and major newspapers relied on individuals who relayed gossip and other news. A recent British case reminded me forcibly of the vulnerability of newspaper reporters.
Reporters have to be generalists by their very nature, and are heavily reliant on their sources.
In a recent edition of a local paper in Britain, a British ultrarunner was credited with having walked a new world best for the 1000 miles, and having set a new End to end record - John O'Groats to Land's End. The claims are unfounded. There was a similar example in Australia earlier this year, when a Sri Lankan runner was credited by newspapers with having broken Yiannis Kouros' 1000-kilometre mark. Over the past twenty years a number of people have claimed to have broken world ultra bests, and their stories have been widely accepted by the mainstream media. When such reports are investigated, tracking down hard facts about times and distances often proves becomes impossible.