It's not just my brain that's not as elastic - my body isn't so elastic anymore either. I can't sit for more than an hour or so before I've gotta get up and take a foam-roller break. Lots of aches and pains that I'm finding are causing me to lose focus.
After ~2 years of not working I found that I didn't much care for it - I needed more purpose or something. Just felt like I was floating along not going anywhere. I think the happy medium would be to work 3 days/week. I could do that indefinitely. Not really be retired and not really be working full time. It would work monetarily as well since the house is nearly paid off and we have no other debt. But software development gigs where you only work 24hrs/week seem to be quite rare.