I usually work in the short sessions carefully planning my compositions and designs in advance, so I would not have to paint over some areas and could have a fresh surface of brushstrokes.
Once I commit to a certain design I attack the blank canvas trying to get as much as possible in the first session.
I believe that the first stage is the most decisive.
I find the process of painting very exhausting physically, but despite all the challenges it is the most rewarding and joyous occupation.