I always preferred flexible hours and still mostly do, however I have a lot of stuff to juggle in my personal life and I found that just allocating a few hours

I always preferred flexible hours and still mostly do, however I have a lot of stuff to juggle in my personal life and I found that just allocating a few hours in 2 or 3 slots in my day helps me achieve more. So I started doing that. Gym's over, I got my meal, rested for an hour, started feeling energized and I am like "OK, time for a 3-4h work session". I make tea and prepare a 2L of water bottle next to me and I get to it. When I feel even little tired, I stop and rest for a bit, maybe eat again. Rinse and repeat. Works pretty well for me during the last year or so. So if you're flexible hours advocate I'd say combine both. You might not work 8h in one sitting but it will still pay off huge dividends to your mental health to have allocated time slots for focused work.