London Mayor Boris Johnson has been wheeled out throughout the year in promotion of the RideLondon cycling festival.
The regular rider will take part in the 100-mile challenge on Sunday.
"My team has told me I will never make it," Mr Johnson said. "They think I am too clapped out to cycle 100 miles but the more they discourage me, the more determined I am to have a crack."
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Two other men missing on Scotland's hills this winter have yet to be found.
The decision was made after two Russian athletes failed drugs tests at the PyeongChang Games.
Sunday will be dry and largely sunny, although there will be some patchy cloud, mainly across eastern Scotland and eastern England.