A survey by sector think tank nfpSynergy found 79 per cent of charities were interested in the idea.
But speaking at the Institute of Fundraising's event fundraising conference last week, Bedford said: "I would imagine 20 minutes in the pub would persuade who-ever had the idea that it's not the best ever.
"The fundraising drive would fall away. The marathon costs £9m to put on. But you can take off the £2m we spend on elite athletes, without which we wouldn't get five hours' TV coverage."
Joe Saxton, nfpSynergy founder, said: "I disagree. The London Marathon did not get five hours' TV coverage when it first started because it needed to develop. You only have to look at how Race for Life has taken off to see the appetite among charities for fundraising through running events."
Saxton hoped a charity consortium would emerge to develop a second marathon.