Good question, Ausra.
The idea of ‘making choices as you go along’ would apply in circumstances where you want to develop some control of your overeating, and I don’t see from your post what is the overeating you would be doing (or used to do) during your working day.
Did you go out and buy something a bit rubbish in those 30 minutes for lunch? If so, then even if you bring your nutritious lunch with you, you can still make your choice in the present time about the other (probably much more addictive) items.
The idea of “Times & Plans” doesn’t mean you never have any clue about what you might eat later on. It’s specifically about your choice around your addictive desire to overeat at the time or occasion or circumstance where you would have overeaten in the past.
As for ‘rigid behaviour’ it’s fine to have some, especially in the situation you’ve described. The issue with rigidity is if you attempt to take control of your overeating with a universal rigidity (all perfect all of the time) and then tend to fall into an “all-or-none” relationship with food, as I address in Week 6 on Relapse.
Let me know if I’ve not answered or understood your question.