The 2004 Olympic bronze medallist was forced to quit through injury in May last year, admitting defeat in her attempts to qualify for London 2012.
But having made a promise to herself that she would complete a marathon in the year after retiring, she is now preparing to run 26.2 miles around the capital.
The 36-year-old, who also worn heptathlon bronze at the 2007 World Championships, had switched to 400m due to injury but returned to multi-events in a bid to reach last years Games.
A prolapsed disc while competing in the 200m during a heptathlon competition in Italy last April put paid to those hopes, but now she is fighting fit and ready for a marathon attempt.
"I always said to myself that I would do a marathon the following year after finishing my athletics career," she said.
"You have to be mad to do seven events, so surely if you can train for so many events then the marathon can't be too difficult!?
"It is going to be my only marathon, but I have been told that when you do one and say 'never again' then you do end up doing more afterwards."
Sotherton, who is being advised by Steve Ingham, the head of physiology at the English Institute of Sport, believes a time in the range of 3:40 hours is possible.
But she is aware of how difficult the challenge that awaits her is.
"I used to get a nose bleed running 35 minutes when I was a heptathlete and I didn't have to do it in the past," she said.
"Doubling the amount of time on my feet now is difficult as I'm not used to it.
"The distance is my biggest worry too, and the bottom line is that I'd like to run the whole way and not have to walk any of it."