Answer: Walking to the park after supper became a habit.
Explanation:
When we omit the necessary information, the two sentences can easily become one.
For example, we don't need to say that we decided to enjoy something, as we never actually can make such a decision. It is either that we enjoy something or not.
To emphasize that we eat supper is also unnecessary, as the supper is a meal and it normally eaten.
There is also no need to use the adjective regular to describe a habit. If a certain action is a habit, it is repeated more or less frequently, so the word habit itself conveys the same meaning as the adjective 'regular'.
