Practice does not makes perfect. Perfect Practice makes Perfect!!
Check how you’re sitting, right now. Is your posture not so perfect? Are you also hunching down?
Your body gets used to this form and this becomes your normal position/posture. Your core gets week, your glutes disappear, etc. There are many studies that conclude that sitting is one of the worst things you can do for your health.
If you work in an office, get a standing desk if you can. Or schedule a 5 minute walk and stretch every hour.
I like to lower my desktop (I place it on a stool) and squat a few times a day for a bit. Or just get up and walk often, sit back, and make sure you do a few drills to straighten up before your run, or your all-day-slouchy-form will catch up with you, literally.
More on Running Biomechanics