Yamauchi has taken unpaid leave from her job at the Foreign Office since January 2006 to concentrate on her running and been rewarded by steadily improving times.
Sunday's result saw her slice almost two minutes from her personal best as she finished ahead Constantina Dita, Catherine Ndereba and Zhou Chunxiu, the three Beijing Games medallists.
"I've been improving for the last four or five years. I knew from my training in the last year I'd make a big improvement and a top-three place was realistic," she added.
Mara delighted with personal best
Yamauchi's more immediate challenge is August's World Championships where she and Paula Radcliffe could spearhead a dangerous British charge.
"I think Berlin will be a super-fast course, which is good for me. Great Britain could win two medals in Berlin - that would be quite a result," she said.
World-record holder Radcliffe, who has not raced at London since her victory in 2005, must first complete her recovery from surgery to remove a bunion that is believed to have been an underlying cause of her persistent injury problems.
"She's a great athlete. I'm taking nothing away from her and just focusing on myself. Paula's CV is totally different from my CV," said Yamauchi.
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