I wonder why "Less than 5% of adults participate in 30 minutes of physical activity each day; only one in three adults receive the recommended amount of physical activity each week." (https://www.hhs.gov/fitness/resource-center/facts-and-statis....) happens?
The answer is that there are so many other things that can stand between "desire drives you" and reality. Just to give you several examples: financial needs, physical or mental health issues, other priorities (kids, family, friendship, etc).
BTW. I am not saying that change is impossible. However, I am not buying "You're on the cusp". Over time you accumulate priorities, which you got by sacrificing a lot 5-10-15-20 years ago. These priorities are dear to your hard. Sacrificing (or even chipping away) from them is way harder than when you are a blank slate.