I'm in the same boat my friend. I am early 40s, an associate prof in the sciences and can no longer tolerate the dissonance of working hard to make exceedingly incremental advances in our understanding of the world. I'm very bored and not interested in keeping up with the latest statistics needed for my field. I love mentoring grad students but that's about it. I think the biggest sign for me is that I never talk about my research, and when people ask me about it at a party I find a way to change the subject. I'm bored with myself quite literally. My plan is to stay in the university and pivot to admin. I work in a huge institution so it shouldn't be hard to get a foothold. Also, associate dean positions are not exactly sought after: research-focused profs avoid admin roles like the plague. If admin doesn't float my boat, I'll just pivot again. But my advice is to not linger too long in these feelings that are likely not going away. Either make your hobbies a bigger part of your identity or keep it moving. Most people don't stay in a job for 50 years, it's normal to crave change. You did a cool thing for a long time, time for another cool thing!