One of my favorite reasons for studying the scriptures is that it allows me to connect scriptures that don't seem related at first in really meaningful ways. This series of scriptures teaches us that we can actually increase our lifetime and improve our quality of life by keeping certain commandments:
20 There is a law, irrevocably decreed in heaven before the foundations of this world, upon which all blessings are predicated—
21 And when we obtain any blessing from God, it is by obedience to that law upon which it is predicated.
This scripture establishes the eternal cause-and-effect between keeping commandments and receiving blessings. What does this have to do with prolonging our mortality?
And the days of the children of men were prolonged, according to the will of God, that they might repent while in the flesh; wherefore, their state became a state of probation, and their time was lengthened, according to the commandments which the Lord God gave unto the children of men. For he gave commandment that all men must repent; for he showed unto all men that they were lost, because of the transgression of their parents.
It turns out that there are certain commandments that, when kept, actually prolong our time on earth as mortals. Here's one:
Honour thy father and thy mother: that thy days may be long upon the land which the Lord thy God giveth thee.
It's worth every effort to increase our mortal time. One reason for this is that it's easier to learn how to bridle the passions of the natural man when we are in our 'natural man', ie. our mortal body.
For behold, this life is the time for men to prepare to meet God; yea, behold the day of this life is the day for men to perform their labors.
And if a person gains more knowledge and intelligence in this life through his diligence and obedience than another, he will have so much the advantage in the world to come.
What about improving the quality of our life?
18 And all saints who remember to keep and do these sayings, walking in obedience to the commandments, shall receive health in their navel and marrow to their bones;
19 And shall find wisdom and great treasures of knowledge, even hidden treasures;
20 And shall run and not be weary, and shall walk and not faint.
21 And I, the Lord, give unto them a promise, that the destroying angel shall pass by them, as the children of Israel, and not slay them. Amen.
...cease to sleep longer than is needful; retire to thy bed early, that ye may not be weary; arise early, that your bodies and your minds may be invigorated.
These chapters detail the heroics of Helaman's stripling warriors. The point I'd like to make here is that it may have been a direct consequence of their actions that secured the blessing of survival and prolonged life. For instance, they
25 And it came to pass that there were two hundred, out of my two thousand and sixty, who had fainted because of the loss of blood; nevertheless, according to the goodness of God, and to our great astonishment, and also the joy of our whole army, there was not one soul of them who did perish; yea, and neither was there one soul among them who had not received many wounds.
26 And now, their preservation was astonishing to our whole army, yea, that they should be spared while there was a thousand of our brethren who were slain. And we do justly ascribe it to the miraculous power of God, because of their exceeding faith in that which they had been taught to believe—that there was a just God, and whosoever did not doubt, that they should be preserved by his marvelous power.
27 Now this was the faith of these of whom I have spoken; they are young, and their minds are firm, and they do put their trust in God continually.