I've done some sort of workout almost every day for the last 25+ years. It had nothing to do with motivation, but making exercise a part of my day like brushing my teeth.
So it isn't about discipline either, it's about habits?
I'm like you. Routine exercise for the last 25 years or so. I feel neither particularly disciplined nor particularly motivated. It is either just a habit I've formed or I happen to have the correct physiological temperament for exercise to be easy.
I took a solid 6 months of forcing myself to ignore my mind's constant excuses to not work out, to the point of my mind realizing that the excuses just weren't going to work and stop.
Exercise feels good to me. I think it felt good immediately when I started? I like the feeling of pushing myself and I kind of like discomfort/pain I can control.
EDIT
I'll also add that most people who are disciplined don't really think about it that way. They just do what they need to do.
I claim the same about hunger (within bounds of moderation etc etc). It's simply uncomfortable, but it's a discomfort that you can adapt to and live with. My general view is that both hunger and exercise work as ways to moderate weight but they rely on the exact same capacity for discomfort (and enduring that requires either necessity or motivation/discipline etc)
It's not much different from starting a business or making a piece of software. Motivation kicks things off because it's fun, but it goes away and now you have to have the discipline to keep going. When people talk about execution is everything, they aren't talking about day 1, they are talking about day 100, day 365, day 720.