I had the exact same experience last year and it fucked me up good with anxiety (which I've never experienced before) and I couldn't eat properly for weeks causing me to lose a stone in weight.
I had the exact same preconceived notions you did with the 1 hour block a day mon-friday. Seemed like it would be a breeze at first and easy money. The problem with this is it just turned into feeling guilty when I wasn't working on it, even outside of the allotted hour. I think I felt like this because there was no clear seperation of environment as I was be doing the work at home and the result was I felt like I was just working all the time.
I did learn a valuable lesson though and that is that my free time is a lot more valuable than I realise.
I will never again consider doing side gigs for anyone other than myself.