Farah, who won the 5,000 and 10,000 metres gold medals at the 2012 London Olympics, will run the first half of the race as preparation for making his debut over the full 42.195 kms distance next year.
"I've noticed I'm 25-1 to finish the race - that ain't going to happen. I'm definitely not going to finish it," a grinning Farah told a packed news conference on Thursday.
The 30-year-old will line up with the rest of the elite men's field in Blackheath but expects to be showered, changed and in front of the television by the time the winner crosses the finish line.
Farah's decision to run only half the distance has drawn criticism, most notably from women's world record holder Paula Radcliffe, who has said she found his decision a bit strange.
"My aim out here is come and learn about the race, get used to the course so next year, I'm ready to go," explained Farah whose main aim this year is the Moscow world championships where he will defend his 5,000 title and attempt to add the 10,000.
"It's a no-brainer really if you think about it, as long as they're giving you a practice run, why would you turn that down?
"There's been criticism, but that's life. You can't keep everyone happy but as long as you do what works for you. This fits in really well and I'm going to learn something.
"Last year (Kenyan) Geoffrey Mutai was in the leading vehicle and learnt about the course.
"It was always my aim to go up to the marathon. I've achieved lots on the track and I definitely want to test myself on the marathon as well," added Farah who won the mini-marathon event three times as a youngster.
Despite the meticulous planning for his full marathon debut, Farah was unsure what would happen once he dropped out.
"All I know is that I'm going up to halfway," he said to laughter. "I'm sure the team will plan something for me."
Farah, who now lives in Portland in the United States to be with his coach, Alberto Salazar, said the bombings during Monday's Boston marathon had not caused him to consider his participation in London.
"It's sad news. My support goes to all the people, the families and loved ones but they would want us to carry on and show our support and continue," Farah, wearing a black ribbon on his tracksuit top in a show of support for the victims, said.
"London is a great city, it's where I grew up, It's where I've done everything, so for me it was never a concern at all. I've got great memories of the London Olympics and what we did as a nation was incredible and I believe we can do a similar job to that (on Sunday)."
- Sports & Recreation