To be joined at the hip means to be closely connected, but in this context the speakrer is describing how hard it was being aloof and absolutely distant to Vanessa in that particular week when he actually got used to being alone.
Consider the statement "Over the past month I had got used to being alone, and spending a week being aloof and absolutely distant to Vanessa working and sleeping with hardly a minute's break, had been hard.". Is the statement "Over the past month I had got used to being alone, and spending a week joined at the hip to vanessa working and sleeping with hardly a minute's break, had been hard." false? Yes/no/unknown