Century Run — Michael W. Davis

Losing his family to an alien invasion was the single most horrific thing in young Lex's life. Joining Omni Corps, an elite fighting force with technology that allows the soldiers to live indefinitely, gives Lex and his team the chance to kill the beasts that destroyed old Earth. But as hundreds of years pass, Lex questions the validity of their missions and begins to long for a normal life.

But Omni Corps is big business, and the bureaucrats want to hold onto power. Can Lex and the members of his squad invoke the Century Run clause of their contracts, or will the people they've been protecting for so long find a way to have them terminated?