He will only make his full debut over the distance in the capital in 2014 - a decision world record holder Paula Radcliffe labelled "a bit strange".
Farah will start with the elite men's field on April 21 and run to halfway alongside the likes of marathon world record holder Patrick Makau, Olympic champion Stephen Kiprotich and world champion Abel Kirui.
The double Olympic champion insists he will not get carried away and run the full 26.2 miles, but close friend Radcliffe said: "Honestly I find it a little bit strange - it's not what I would have done.
"Either you find a good half marathon somewhere or you take the plunge and attack the distance and race it. Here he's caught between two stools."
Farah actually will contest a half marathon in New Orleans next weekend - quite a change in distance after claiming a comfortable win over 3,000 metres in seven minutes 42 seconds at the British Athletics Grand Prix in Birmingham on Saturday.
The 29-year-old, who won Olympic gold over 10,000 and 5,000 metres in London last year, said: "I think practice will make perfect. The more practice you can do you can get it right.
"And running in the London Olympics and having 75,000 people cheering for you was the best thing ever. I get excited just thinking about the amount of people who can come out there (for the marathon) and give you that support and I think that's what the sport needs.
"Hopefully I can give back something to the crowd and everyone who couldn't come to the Games."
Race director Hugh Brasher estimated that up to 650,000 people watch the London Marathon on the capital's streets, but would not reveal what Farah will be earning from the race.
"As a double Olympic champion, Mo rightfully will be well rewarded," Brasher said.
The news about Farah somewhat overshadowed the action at the National Indoor Arena, although there was a fifth victory in six competitions in 2013 for Holly Bleasdale in the pole vault.
Bleasdale won with a first-time clearance of 4.70m and narrowly failed to improve her world-leading mark of 4.77m, just brushing off the bar with her last attempt at 4.78m. Olympic silver medallist Yarisley Silva of Cuba was second on countback.
"I think if I can clear 4.70m first time at the European Indoors I have a good chance of the gold medal," Bleasdale said.
There were also British victories for Nigel Levine in the 400m, Shara Proctor (6.78m) in the long jump and Michael Rimmer in the 800m, although Rimmer had to survive a protest from Sudan's Abubaker Kaki, who was sent sprawling across the line from an apparent push by Rimmer in the closing metres.
"I had nowhere to go. I had to get close and he dived in front of me," Rimmer said. "I had no choice but to put my hand out. I didn't push him even though it looked like it."
Helen Clitheroe could defend her European Indoor 3,000m title in Gothenburg after winning inside the qualifying time, but the 39-year-old said: "Training is planned around the marathon. I'll see what my coach says but I'm doing a half marathon next Sunday so now my focus is on that."
Jenny Meadows also faces a decision about whether to compete in Gothenburg after achieving the qualifying time when finishing second in the 800m - her first race for more than a year following injury problems.
Robbie Grabarz beat Olympic champion Ivan Ukhov in the high jump, but still finished second with a best of 2.29m after another Russian, Aleksey Dmitrik, was the only man to clear 2.33m.
Tiffany Porter was third in a blanket finish featuring six athletes in the 60m hurdles, but Dwain Chambers failed to make the final of the 60m and faces an anxious wait to see if he gets selected for the European Indoors.
"We'll just have to see what happens with the selectors," said Chambers, who had missed two weeks of training with a back injury. "But I will only want to go if I am 100 per cent. My only aim is to win a medal. They have to make a calculated call and if I'm not fit then there is no point selecting me."
Greg Cackett was fifth in the final in 6.66secs - outside the qualifying time of 6.60 - with Harry Aikines-Aryeetey seventh.
In a high-class women's 60m, Murielle Ahoure of the Ivory Coast lowered her own fastest time in the world this year to 6.99s, to win ahead of double Olympic 100m champion Shelly-Ann Fraser-Pryce, who clocked 7.09s in her first indoor meeting.
Ethiopia's Genzebe Dibaba failed in her bid to break the world record over 1,500m, finishing just over 2.5s outside the mark after a solo run from halfway.