Moon Knight points out that Marc Spector has been a superhero for a long time in the MCU, and may have become Moon Knight during Avengers: Endgame.
Mark Spector is the newest superhero in the Marvel Cinematic Universe, but Moon Knight makes the timeline of when he was a Ranger unclear. During the MCU, Marvel Studios traditionally introduced new heroes to audiences by showing them their original stories. This is especially true when that character leads a solo project just as Oscar Isaac's Moon Knight does now. However, Marvel breaks its formula in this case by proving that the Moon Knight story isn't the first time Marc has served Spector Khonshu, the Egyptian moon god, to achieve justice.
The emergence of the MCU for Moon Knight comes on the heels of Avengers: Endgame and the Infinity Saga. This Easter egg confirms when Moon Knight happens after the surprise Hulk reverses Thanos, thanks to a vision of the World Replay Board featured in The Falcon and The Winter Soldier. While the Disney+ series follows the Moon Knight adaptations of Stephen Grant and Mark Spector in the period since these events, it's also been emphasized that this isn't the first time Moon Knight has seen action. Episode 2 of Moon Knight includes multiple references to past missions and an unspecified, but the long, history between Mark and Khuncho.
It's the result of Moon Knight's mysterious story and lack of MCU connections that raise questions about how long Marc Spector has been in Moon Knight. Mark can't remember exactly how long he had been serving Khonshu when Stephen asked him, but he knows it's been a long time. This comes after Moon Knight Episode 2 teased that Mark Spector survived the Thanos take and indicates that the story takes place in the year 2025. However, it was not confirmed when Mark Spector became Moon Knight. Kancho alludes to Mark's imminent death which paved the way for him to become the avatar of the deity, but it is unclear when exactly that happened. The catch, however, is that this is not a recent development in Marc Spector's life, suggesting that he was a Moon Knight before Phase 4.
In terms of narrowing down how long Mark Spector has been on Moon Knight in the MCU, his passport issued in late 2018 could provide a clue. His passport may have been updated before his final mercenary mission which led to his near-death and his relationship with Khuncho. This likely means that Mark Spector became a Moon Knight during the chaos that followed Thanos' surprise. If this is roughly when Knoshu approaches Marc to make him Moon Knight, that gives him nearly six years of potential adventures with Khonshu. Marvel Studios could reveal that his superhero career is a bit longer or shorter than currently expected.
If Marc Spector has been a Moon Knight for several years in the MCU timeline, that means he was active during the events of Avengers: Endgame. It still makes sense that he wouldn't have come to help fight Thanos' army in the end because he likely never came across Wong or anyone else who would have known that he asked for his help in the end. However, this also opens the door to crossover opportunities in the Moon Knight's past. Marc Spector's Moon Knight could have crossed paths with someone like Clint Barton during Ronin's days or even dealt supernatural threats with Blade. It remains to be seen if Moon Knight features or mentions any of these adventures, but Marc's proven past as a superhero gives Marvel Studios more options going forward when it comes to integrating him into the larger universe.