Yes, this is a difficult subject.
Let's look at what we do know. Original man (before Noah's flood) lived to tremendous ages. Adam lived 930 years, Methuselah lived 969. But after Noah's flood, the lifespan of man decreased very rapidly, although men still lived long ages compared to today. Abraham lived 175 years, Moses 120.
What happened? I think it was a change in the earth itself that made a difference. Before Noah there is no rain mentioned in scripture, only that a mist went up from the earth. There seems to have been a canopy of water above the atmosphere that broke open in Noah's day.
Gen 7:11 In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.
It is interesting that this verse says the windows of heaven were opened. A window is clear, you can see through it. We open the windows of our house to let stale air escape, and let fresh air enter.
So, there seems to have been a great change in the environment. Men lived to old ages, it is probable animals and plants did too. This could account for the tremendous size of the dinosaurs as most reptiles continue to grow as long as they live.
And I believe when the scriptures say there were giants, this does not mean men only. We have found fossils of gigantic insects. A dragonfly fossil having a wingspan of over 2 feet has been found.
Gen 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Is this speaking only of men? Not necessarily. Notice right after it says there were giants in the earth in those days, the next phrase starts with "and also" which means it is speaking of a new subject. So I believe people, animals, sealife, and insects lived to an old age and attained giant size.
So, life span seems connected to the environment. Science has said for many years that man should have the ability to live indefinitely, and they are not sure why the body shuts down and ages when it has the ability to regenerate itself.
The tree of life must have contained whatever nourishment the body needed to keep regenerating itself.
This is all speculation on my part, but it seems man originally was designed to live to extreme age. God had to prevent man from eating from the tree of life or he would have lived forever.