Hi Claire, 

im so sorry to hear about your dad, and I can absolutely relate. My grandad (72) who is like my father, has also just been diagnosed with stage 4 lung cancer, which has spread to his lymph nodes. He is also completely symptomless and looks just like his usual self. He starts his chemo on Tuesday and is having two rounds before having a scan to see if he is responding to treatment. He has been given a diagnosis of life expectancy with and without treatment. 
I hope you and your dad are coping ok. It has taken me 13 weeks (that's when he was diagnosed) up until the last few days to accept what is happening. Even then, I still can't believe it. Lung cancer is a very unforgiving disease and 90% of people are only diagnosed after it has spread. Sending lots of love and positivity your way x