Izzard - who ended his mammoth challenge in the pouring rain - said he had been trying to complete his final day's run in under five hours but had failed to do so by 30 seconds.
"I don't know what the hell I was doing at the end but it must have been way faster than I was doing the rest of the time.
"So I feel exhausted now, but you know, it's over."
He added: "Being here is very nice because, when I left here seven-and-a-half weeks ago, there was nothing here, it was just a cold morning and now there's a lot of people here, even in the rain."
The comedian, who has so far raised more than £200,000, was joined by well-wishers for portions of the challenge, which took in England, Northern Ireland, Scotland and Wales.
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