It’s a risk hedge, it has social benefits, and it has capacity / functionality benefits.
Risk hedge: If AI doesn’t kill us, there will be a time lag between now and when we can transform ourselves/obsolete our biology. Maintaining your current biological hardware increases the odds of getting to the transformation state. If you’re old (like me), this matters a lot.
Social benefits: Being in shape changes how you look, and that improves how other people treat you. Depending on how social you are, this may have a lot or very little impact.
Capacity / functionality: If you can run a mile, you can walk a mile without getting tired. You can climb stairs more easily, you can rush across an airport to catch a flight, you can carry your groceries in one trip instead of two. It’s like the saying goes: if you’re deadlifting 250 pounds, you’re not going to throw out your back picking up your kid.
For me, the big one is hardware maintenance until I can upload. Uploading is at least two decades out (probably more like five). My odds of getting there are materially better if I put effort into hardware maintenance now.
This pretty much checks out with my experience. Boring and bland is annoying, but the alternative is eating an entire box of lucky charms in six hours, so boring and bland it is.
I don’t do it because it’s fun; I do it because it’s necessary. This (biological) hardware sucks so much.