I don't have a business, but I have been working on a side project for almost 5 years after hours, I commit about 20 hours a week to it. At the beginning I tried what I called 'super days' that sound a lot like yours. What I found is that while I was 'working' those 5-6 hours at night I was not very productive mostly due to mental exhaustion. Then on 'off days' I was way too exhausted to do anything, and I never fully recovered. For my project (creative in nature) this caused crippling Creative Productivity Anxiety and depression.

About 1.5 years into the project I broke my foot (long story) which meant I had to change things, then I had a kid about a year later, which caused me to change again. Both of those were really good developments

What I found is that a far better schedule for me was to spread things out. Now I still put in about 20 hours a week, but it is in the form of 1.5 hours at night on week nights and 6 hours on Saturday and 6 hours on Sunday. The Saturday and Sunday time is generally split three ways (morning before family wakes up, kid's nap time, evening after kid goes to sleep) I feel like I get more work done because I have the mental ability to actually work for each of those pieces of time.

Schedule may look different for you, hobbies, family, etc. But I would definitely suggest with exploring smaller pieces of Second Work, especially on week days.