‘Discipline’ in the context of this post requires a certain level of mental effort, also known as ‘motivation.’
The problem with this approach is thoughts and emotions ebb and flow like the weather, so waiting for motivation puts us at the whim of things largely outside our control. Also, ‘trying’ to control such stasis tends to put us back at square one mental effort-wise.
‘Lifestyle’ on the other hand is about making something a part of our daily/weekly lives. Stephen King for example writes four hours a day, and reads four hours a day. Everyday.
Again, life is an experience not a contest, so the time we allot to given activities are naturally going to be different for everyone. The larger point is making the things we enjoy and the corresponding activity growth in these fields requires, as much a part of our lives as brushing our teeth.
Simplicity, simplicity, simplicity.