Walker's admission came at the retailer's annual conference, attended exclusively by The Grocer.
Iceland has enjoyed average like-for-like sales growth of 9% over the past five years since Walker and his management team, who were ousted in 2001, were reinstated in 2005.
But in the first 28 weeks of the current year like-for-likes were up just 2.5% and Walker warned that growth would be harder to achieve now the business had been fixed.
"Looking at it now we could be forgiven for saying we can relax," he told more than 1,000 managerial staff at Birmingham's ICC. "Store sales have gone up from £37,000 to £65,000 a week, profits have gone from zero to £180m-plus. We had a lot of debt now we are debt-free with £80m in the bank. We are financially one of the strongest retailers out there. In years three and four the business was flying. It became easy. I started skiving I sailed across the Atlantic and a bit of the Pacific. But it won't continue forever. Complacency is really dangerous. It could all go wrong tomorrow."
However, Walker and his team have such confidence in the future of the business they are trying to buy it outright, having put in a £750m offer to buy Landsbanki and Glitnir's 75% stake in the retailer valuing the company at £1bn.
Although the nationalised Icelandic banks had not accepted the offer, the deal was still "on the table" and Walker was confident they would take the money sooner or later because they needed to repay their debts.
To help boost like-for-likes, store managers were urged to raise standards and make customer service a priority. And in the next financial year managers are to be incentivised with the prospect of a bonus of up to 40% of their salary.
New ranges are being brought out for Christmas, including exclusive party platters and desserts, backed by an £8m marketing push.