In section 2, the speaker coveys that he or she is 20 years old when saying that "Twenty will not come again" like in a nostalgic way, recognizing the time that has passed and that he or she would never get back, and then the speaker sets the age of 70 as the top age that they will probably live in total, which leave them with only 50 years left to live, however they consider them not enough years. Life is too short.
In the following section, the speaker again recognizes that 50 years left is too little to fully be able to enjoy life and nature. In any case, he or she has the intention to enjoy it.