I think the "motivation" that people consider fleeting is actually inspiration (like when you have a new project idea and stay up overnight excitedly working on it).

Discipline is important, but without motivation you will end up doubting why you are working so hard on something you don't care about. In my mind, motivation is continually reminding yourself of what outcomes you're working towards, so you don't lose sight of why you're working so hard.

For example, motivating yourself to work out by thinking of that sporting hobby you'll be able to take up or thinking about how great you'll feel when you've reached your goal weight.