Perseverance may not be quite the right word for this theme in Endgame, and you'll find that at other points in this guide, we use the word "endurance." The thing is, perseverance implies the possibility of success if you just keep going. There is an obstacle, and you are going to overcome it. In Endgame, this is not the case. There is no way out of the apocalyptic struggle, but the characters do suffer through it nonetheless. If there is any perseverance, it is mental rather than physical. Their bodies are deteriorating and they will die. Yet, even when the characters seem to have given up hope, they still struggle on for reasons that they cannot comprehend.
In the play, perseverance and defeat are the same thing. Real perseverance would be working up the courage to accept defeat and bring about an end to this horrible way of life. When the characters seem to be enduring, they are actually just revealing their weakness and inability to admit that they are defeated.
Hamm is the only character in the play that actually endures anything. His behavior is self-determined, whereas Clov only takes orders. By contrast, Clov is not demonstrating endurance. He is demonstrating inertia.