At what mile marker of a marathon are you finishing no matter what? Aches. Cramps. Heck, maybe even broken bones. Twenty miles? Twenty-two? Twenty-three? This is where the Mets are with Matt Harvey. They have come so far and through so much with him that it makes no sense to end the marathon now. He is a free agent after the 2018 season. I think he could go 22-0 with a 1.00 ERA and the Mets wouldn’t have him back. But unless they feel he is a totally lost cause, then why not finish the marathon? And why would they have tendered him a contract for 2018 that will cost about $6 million unless they had some hope? The Mets have had greatness one time in Sandy Alderson’s term, when Harvey, Jacob deGrom, Steven Matz and Noah Syndergaard were concurrently healthy to fuel a run to the NL title in 2015. Nothing changes much now. No matter what the Mets do around the edges of the roster — like coming to agreement Wednesday with reliever Anthony Swarzak — they will again be beholden to how much talent and health they can get from their rotation. The surest things — a term you hate to use when it comes to the Mets and, especially, pitching — are deGrom and Syndergaard. Harvey is part of the wild-card group that will determine if the Mets are good enough to play for even, well, the wild card. “The stuff is still there,” new pitching coach Dave Eiland said. “Health is going to be the key. If he stays healthy, then he’s a solid part of the rotation.” A solid part of the rotation is a step or three down from the next Tom Seaver, but where do the Mets sign up for that? The Mets have this going for them: If Harvey is not going to give them his best shape and behavior now, then when? He is entering his walk year and already has crushed the chances of a $200 million free-agent payday with questionable health, poor results and a dubious off-the-field life.