I would add to Eric's comment that even if you can do back-to-back 1,000-mile days for as long as you want, there may come a time when you cannot do half that in one day. Illness, medication, stress, or just plain having a miserable day can lead you to the point where you cannot safely complete the ride, or you start asking yourself "why am I doing this?"
Case in point, I've completed 4 Iron Butt Rallies and more 800 - 1000-mile days than I can count. As a ride in to the IBA banquet in Jacksonville a few years ago I was planning on a SS3K. I left Philly in rain, my right heated glove quite working and as I rode south on I-81 temps got colder rather than warmer until I hit TN. Near Nashville I rode through a thunderstorm with imbedded tornados (found that out later) and my Blue Tooth headset quit working. Stopped for the night east of Nashville at just under 1,000 miles, and when I left on day two my headset was till not working, I was miserable, and tired to the point that I was pulling over every 20 minutes for a short break. It started raining again as I entered Oklahoma and when I pulled into a rest stop for a break, I had the revelation: "Why am I doing this if it's not fun?" I rode the 20 miles to the closest hotel, took a shower, and slept until 11am the next day. That was the first IBA ride I did not finish due to non-mechanical issues.
Lastly, I also bailed on the Hoka Hey in 2021 because of internal stress caused by some family health issues.
The moral of this long-winded answer is to know when to say when as Eric stated, this is supposed to be fun and if it isn't then it's time to reevaluate what you are doing.
Other than that, be safe and have fun!