The older I get, the harder I find it to have those deep focus work sessions.
Today I Googled "how to get into the coding zone" and got this interesting Stack Overflow thread. All of these answers are interesting, but I have my own view here:
To get into the zone, don't think about getting into the zone.
It's same as going to sleep. If you want to fall asleep, you don't think about how much you wanna fall asleep and you don't ask "am I asleep yet?". Falling asleep often involves thinking some easy thoughts that soon become the dream.
To get into the zone, just focus on the problems you are solving and don't make a big deal out of it.
Just take it easy. It will come.