It is often said that the greatest shortcoming of the human race is our inability to understand the exponential function. It is worth tracking how most people also vastly underestimate even the linear function, when it keeps running over a long period of time. For instance, 20 minutes a day is already more than 2 hours a week already, 10 hours a month, and 120 hours a year! You can make a decent bit of progress in just a few weeks for most things at that rate.
I believe that even the linear function is not well understood because having a long time horizon is not common. Most people don't do things over 7, 10, 15 or 20 years, so they never experience how radically different their life can become from small habits. Walking every day for 20min doesn't sound like it could produce significant results, and yet, when you think about all the following benefits:
It could be the difference between gaining or losing excess weight, which compounds to a lot over a long time horizon
It prevents you from doing harmful things to your life, by virtue of being outside, for instance getting sucked in addictions, or overeating, or making yourself miserable with the internet, etc.
It helps regulate difficult emotions, feel more in tune with your body
It provides a breath of fresh air, literally, and sunlight
It gets you out of your head, which in many ways is a dangerous neighborhood when it is the only thing you have
It potentially opens up the gate for other things: meeting new people, more physical activity, a newfound appreciation for life
Good habits build up pretty significant abilities and open up new pathways in a matter of months, and within a few years you've become in many ways a new person. In that regard, it is somewhat of a shame that we tend to judge our life trajectory based on feeling better than the last day, as opposed to carefully draw out trends that span weeks, months or even years. Change takes time, and even on a positive trend there are bad days. As such, the people who understand linear returns over a long period of time are quite rare.