Before you start writing a book, accept that the undertaking is a marathon, not a sprint. It will require time, imaginationa dn dedication. There is no magic formula, writing a book and maintaining a job at the same time can feel like trying to pat your head while rubbing your stomach simultaneously.
So I would advise that you make sure you enjoy your writing. There were certain times when I really felt like I should write but I did not feel able to. I was too tired and so I did not write on those occassions. Listen to your body. Stop patting your head, provided this only happens from time to time your book will still be written on schedule. Your day job will be grateful to you.
Now, I will try to read your mind! I hear you thinking, but what if I listen to myself too much and never sit down to write? Good question. Make yourself an achievable goal. Make a plan you can work with and never set an unrealistic target.
I have often told myself that I must write until I have finished the chapter I am working on, or reach a certain word-count. Only, after seven precious weekend hours, did I succeed, but I was exhausted, and could not bring myself to write anything else for days. To be honest, the chapters written that way are often edited out in their entirety, probably because I was more concerned with typing words to meet targets than to inspire readers or create a fine story.
So, you need to find a balance - a work/write/life balance is the key. While you are working fulltime at a day job, you must enjoy your writing but also give yourself time to do other things apart from work and write. Let yourself enjoy your writing but know that you must get time off to do other things. Without this, you might end up resenting your writing and to resent something you love doing would be very sad.
So do set those realistic goals and make sacrifices. Give up evenings or weekends but not both, otherwise you will get too tired and fed-up. After all, you never want your writing to be tiring, you want it to be inspiring.