Sandy Alderson wants the challenge. The Mets general manager said rebuilding this team after it bottomed out is something he wants to do and feels a responsibility to do. So Sunday, after starting with the cosmetic change of having Terry Collins step down as manager, Alderson said he appreciates the chance to try again with the Mets. “It’s been a disappointing season, I think we have more in us. I personally take a lot of responsibility for unmet expectations. I am happy to have the opportunity to try to correct that,” Alderson said, confirming he will be back in 2018. “Any season is a challenge. A season following a season like this one will be challenging.” The Mets are expected to soon announce that Alderson, who turns 70 next month, will be back on a two-year deal. While that leaves open the option to continue with the Mets beyond 2020, assistant GM John Ricco has been positioned as the obvious heir apparent. But the Mets wanted a steady hand during a period where they are selling change.