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Josh Griffiths, 23, finished in two hours 14 minutes 49 seconds on his marathon debut to claim an automatic qualifying place for August's Worlds.
Griffiths, who started behind the elite field, was the first British runner to cross the finish line and 13th overall.
"The crowd was amazing and carried me to the finish line," Griffiths said.
"The goal for me was to try and run the Welsh Commonwealth Games qualifying time of 2:16:00. It never crossed my mind that I would be running in the World Championships in the summer."
Robbie Simpson was fastest of Britain's established names, finishing in 2:15:04 ahead of Andrew Davies (2:15:11).
The race was won by Daniel Wanjiru of Kenya in 2:05:56.
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"I didn't really realise until I crossed the finish line what I had just done," Griffiths continued.
"I managed to catch up to the lead British runner at about mile 13, but I was working hard with a group of really elite runners who I have looked up to for a long time.
"Before I knew it, I started to pull away from some of them. I was working hard with Robbie Simpson at mile 22 and I knew that my legs were starting to tire.
"Around mile 25, I started to think I had passed everybody, but you're never really sure until you cross that line."
Griffiths, who finished third in the Cardiff Half Marathon in October, says he was aided by the pace of Simpson.
He added: "I've known Robbie from mountain running previously and I know he's a great runner so the fact that I was able to run with him and work together with him was just great.
"He's really strong and I think we helped each other get to the end.
"This is all very new to me and the thought of being able to run in the World Championships in the summer, I just can't wait."
A club runner with Swansea Harriers stunned Britain's elite men at the London Marathon to qualify for the 2017 World Championships in London.