Farah participated in the Vitality London 10,000 this week, marking his return to action since his failed Tokyo qualification last June.
The 39-year-old battled through a foot injury during the event, and ended up finishing a minute outside his best time on the road - a time he set around the London course back in 2010.
But while Farah struggled with his foot, 25-year-old Ellis Cross kicked on and crossed the line first.
After a frankly unimpressive outing, Farah was questioned about his future plans in running, and the Olympic gold medalist refused to confirm any events he was training for.
"Your body has to be ready. You have to be in the right frame of mind. You have to be able to compete with the guys.
"Today was a tough day. Ellis did really well to win here. But you've got to see where you are. At the minute I don't even know. You've got to take it race by race and let the body see what it can do. I'm not getting any younger, am I?"
Cross and Farah both stuck with the leading group early on, but ended up breaking away from the pack - which included 41-year-old Chris Thompson.
But while Farah was able to stick with Cross for much of the distance, the 25-year-old finished well to record a final time of 28 minutes and 40 seconds, while Farah came home second four seconds later.
"I did put some good blocks of training together but it's been so, so long - nine months since my last race," Farah added.
"I love the sport and I love what I do. I've had a long career but there's a lot of youngsters who are coming through, which is nice to see."
After upsetting the odds and claiming the win, Cross suggested that the win was "overwhelming".
"I didn't believe it until 20 metres from the finish, I was like, 'He's just going to outkick me, that's just that'. I can't explain the feeling. It's overwhelming.
"The most inspiring thing for myself is I've beaten somebody that's an idol. He's known around the world and I've just beaten him over his prime distance. I didn't quite follow the script today but, to pull it off, I'm delighted."