Unable to work for more than 4 hours in a day

Alright so this post is coming from a place of guilt so do not take it as bragging. I have been employed for more than 16 years now and routinely clocked 60+ hour work weeks till before COVID. Once WFH started, I found it difficult to adjust working from home (especially with two toddlers) and found myself spending most of the time taking care of the kids (which I thoroughly enjoyed). Wife is a doctor and has erratic hours so I thought kids need the reassurance of at least one parent being around most of the time. Around 2021, I realised that my fitness has significantly gone down so I enrolled in a sports club and began playing squash regularly and religiously in the mornings. 30 minute sessions became 45 minutes which became 1 hour to now close to 2 hours (coupled with some running and swimming). By the time I login to work (I have UK hours: 11 am to 8 pm), I feel quite tired and at times take post lunch naps (during office time). My health has significantly improved but I have found that I am simply unable to stay productive at work for more than 4 hours ( around 4 pm to 8 pm). I am pretty good at what I do and one of the few technical people in a team full of Excel worshippers so one can argue that I get things done relatively faster than average colleagues. However, I have a serious fear that this level of slacking may get me in trouble and could even render me unemployable as I get older. Is this just a phase or a manifestation of a midlife crisis? Have others experienced something like this in their late 30s early 40s?