The Flora Family Marathon was dreamt up by the brand to celebrate its 10 years of sponsorship of the main race and the 25th anniversary of the original marathon. It was launched on 17 January.
The idea is that families pay £10 to run the race in stages over five weeks, in support of charities that have been picked by four celebrities.
The final leg takes place on the day of the London Marathon. But a spokeswoman for Flora Family Marathon admitted: "So far the sign-up hasn't been as good as we hoped. It has got off to a slow start."
She rejected the suggestion that families were put off by not being able to choose their own charity, and insisted that race organisers remained confident that they would manage to fill the full complement of 20,000 runners.
The four celebrity endorsers, who will run the race with members of their own families, are Sir Steve Redgrave, Gordon Ramsay, Lorraine Kelly and Blazin' Squad. Each celebrity team hopes to raise £50,000 for their charities.
- Flora is organising the Flora Family Marathon
- Sir Steve Redgrave's chosen charity is his young people's foundation
- Gordon Ramsay selected baby charity Tommy's
- Lorraine Kelly chose MS Society Scotland, Scottish Heart and Arterial Risk Prevention, Maplewood School, and Scottish Society for Autism.