The Briton, who built up an aura of invincibility after winning 5,000 and 10,000 metre titles at the last Olympics and world championships, said he was disappointed to finish eighth, nearly four minutes behind winner Wilson Kipsang of Kenya.
"I will be back. 100 percent. I'm not going to finish on a down. I wanted to give it a try," he said of his decision to take on the greatest marathon line-up ever assembled.
"London, this is my city. This is where I grew up so it would have been wrong for me to do any other marathon.
"It was a great experience to race in London. For me I learned a lot... I know what marathons are about now. I'll come back stronger.
"As a champion, I've won gold medals at Olympics and worlds - that's as big as it gets. The marathon is a challenge - I want to be able to know I can run a great marathon."
Used to controlling races from the front and winning a flurry of track golds, Farah, 31, came into London as a novice, stepping into unchartered territory.
While Ethiopian long-distance great Kenenisa Bekele made a successful marathon debut last weekend in Paris, Farah chose to mix it with the elite of the sport, with four of the 10 fastest men in history on the start line.
The fastest of all - Kipsang, who set the world record 2:03:23 in Berlin last year - blazed to victory in a course record 2:04:29 with Farah clocking 2:08:21.
A measure of the calibre of men Farah was taking on was reflected in Ugandan Stephen Kiprotich, the Olympic and world champion, finishing behind Farah in 12th.
"I'm quite disappointed today in terms of my time and where I finished. But if you don't give it a try you're never going to know," Farah said.
"It's just the way it is, no point thinking about it too much. I had a bad day in the office. Got to move on... just have to get ready for my next race, have a bit of a break."
Farah opted to run in the second rank from the start, giving the leaders about a 40-second advantage, a gap that only increased from halfway as the Briton increasingly found the going tough.
"The leaders started at world record pace and that was never my intention," he said.
"I started to feel tired. It hit me between 17 and 18 miles. I slowed down a bit at that point, that's when I started feeling it.
"My upper body wasn't too tired but my legs were getting heavier and heavier."
Reflecting on his preparations, Farah said everything had gone to plan, despite collapsing at the end of the New York City half-marathon last month, a mishap he put down to the cold conditions in the Big Apple compared to his warm-weather training at altitude in Iten, Kenya.
"I did everything that I could...been out in Kenya, had my coach alongside me, had the team around me so I don't think I could have done anything differently."
Former British distance runner and now BBC commentator Brendan Foster said Farah should forget about the marathon until after the Rio 2016 Olympics.
But Farah said his marathon dreams were far from over although he was unable to say where and when his next one might be.
"If I could come back in two weeks' time I would. Because that's the kind of athlete I am. I never like to finish somewhere so low. But I have to make a sensible decision. I haven't thought much about it. In my heart I do want to come back but I don't know when.
"I'll have a little bit of time off. Relax and enjoy time with my family because I've been away for the last three months. Just forget about running."
- Sports & Recreation