You bring up a very good point: playing long games requires stamina of mind and body, as well as some coping techniques for nervousness, focus, etc. At one point I began to dread OTB play for precisely the stresses that you describe, and others. Even local club games were G/120 (or 4 hours in duration) and I could hardly manage the nerves and attention span needed. I remember one particular regional event I played in. It would be a 5-round tournament with time controls of 40/120 and SD/1 (first 40 moves in two hours and sudden death in 60 minutes after that) - the possibility of a 6-hour game ensued. Friday night. Round 1 started at 7PM. Would you know it, but my opponent and I would be the LAST pair playing, finishing at 12:55 AM! Worn out already, with 4 more such rounds to go. I don't remember whether I won, lost, or drew. Alas, I eventually gave up OTB chess due to such events. Others were able, willing, and even interested in such battles. I was not.
So, I really don't have any advice, just a sympathetic ear from one who struggled mightily with long games. I have more such stories but won't bore you with them ... and if I continued, you might never play again.
Congrats on having completed your tourney; good luck in the future.
I restarted playing about 4 months ago and there was a fairly local tournament, so I thought I'd enter. Well I did manage two wins out of five rounds. Two of the losses were more about the time, not that I was running out, I just couldn't deal with these tense closed positions after 80 minutes. That's also about the time I won the other two games in. So one lesson is that I don't have the stamina for 2 hour long games
I also noticed that given these time limits the games were more precise, positional, and closed than a lot of what I see online, or even playing for fun. I think usually my games have decent openings and then it's thinking 2-3 moves out, combining basic tactics and positional thought. But, I think I need more focus on specific targets a la Nimzowitch. But 2 hour games one after the other, how do people prepare?