Done! 
My do-si-do is done! Yesterday I rode nearly 32 miles; and this morning I put in 30 more. There’s my back-to-back long rides. Not that 30 miles is so very long stacked against the daily mileages of RAGBRAI; but it’s a beginning.
Today’s ride was harder than yesterday’s, which is no surprise. I’m not used to doing 30-mile jaunts two days in a a row; and the wind was stronger than it was yesterday, too. But I got through without feeling like I was dying; and that’s important.
This do-si-do did point out the importance of improving my pace. I think I’ll work on my pace Wednesday; tomorrow I want to take an easy ride to the farmers market. My legs need to recover from these two days of long rides.
What's Next? 
What’s next, now that I’ve done back-to-back long rides? Obviously, keep working up to longer do-si-do riding. Keep trying various electrolyte drinks and foods. As already stated, work to improve my pace. This morning I averaged a plodding 9.18 mph; and at that un-speed, I couldn’t get through a 71-mile RAGBRAI day on time!
How soon is too soon for another shot at back-to-back long rides? Is once a month too much? Too little? Just right? Maybe I should go ask Goldilocks.
But if I’m aiming for RAGBRAI 2025, I still have plenty of time to work up to it; and to figure out what’s next.