Cain and Abel were not Adam' first offspring.
Yes, according to the scripture account, Cain and Abel were Adam and Eve's first offspring.
Gen 4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
Gen 4:2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
The reason Eve said what she said about Cain, is because, as the firstborn, she would think that he (Cain) was the fulfillment of the promise of Genesis 3:15, "[the] man" (thus the "seed", as Cain is a tiller of the ground, a sower of seed), the redeemer. Eve truly thought Cain was the promised deliverer. Turned out he was a murderer instead.
Additionally, Cain and Abel are the only ones that bring an offering (as taught them by Adam/Eve), and this further indicates that there are no other sons around, though Adam and Eve did later have more children after them (Genesis 4:15,17, 5:3-4). Please take notice that the text does not say that the sister that Cain later married was alive at the time of Abel's death.
The words are indicative of both Cain and Abel being Adam/Eve's first children, with Cain being firstborn, followed at a later period by Abel. Even the Jewish commentaries agree on that point, and the talmud, the midrash (Bereshit Rabbah 22:7 - "Said R. Huna: An additional twin was born with Abel, and each claimed her. The one claimed: ‘I will have her, because I am the firstborn’; while the other maintained: ‘I must have her, because she was born with me'”", etc) also.
People read the bible like God created Adam and they fell by lunchtime.
No, I don't. I read it as it says, being naturally (1 Corinthians 15:44-46), as opposed to unnaturally and forcing all kinds of non-scriptural material into the text.
The Bible does not give a specific time as to when Adam and Eve fell, but we know that it was before they had "Seth" which was when Adam was 130 years old (Genesis 5:3-4), and before they had Cain, and Abel also (Genesis 3:1-24, 4:1-2). So in general, we know from scripture, it was before Adam was 130 years old, at least, and more so that Cain and Abel were old enough in the history to make offerings, and keep responsibility (Genesis 4:3, "process of time"). We might also attempt an average among the first genealogy in the births of Seth's line and give a possible estimate for when Cain and Abel might have been born (but it would be speculative at best). For instance, Seth was 105 (Genesis 5:6), when his first child was born, and Enos was 90 (Genesis 5:9) when his first child was born, &c. Mahalaleel and Enoch both had their first born at 65 years of age (being the youngest listed) (Genesis 5:15,21 respectively).
So, no, they did not fall "before lunchtime", as the text indicates that time took place between their (Adam/Eve's) creation and their fall, for why would Eve be so early deceived after being warned of God and angels of the deceiver that had been cast out of Heaven? The serpent was "more subtil" (Genesis 3:1; meaning, biding its time), and waited for the right opportunity to then strike.
They could have been in the garden millions of years before the fall.
Absolutely not. The Bible is crystal clear on this. Adam's age, in total from first day of creation until his death, was 930 years (all), see Genesis 5:3-5. It says "Adam lived" 130 years and begat Seth. It says, "all the days that Adam lived were nine hundred and thirty years: and he died." (Genesis 5:5). It is an impossibility, according to the text of scripture, for Adam or Eve to be any older (like thousands, millions, billions), for Adam himself, never made it to a full "day with the Lord" (2 Peter 3:8, meaning 1,000 years, but died just short of it, according to the promise of God, Genesis 2:17).
That's how Cain was able to go to Nod.
"Nod" just means a place of wandering (for he was to live as a "fugitive and a vagabond" (Genesis 4:14), in the valleys below and outside and to the east of Eden/the Garden of God on earth (Genesis 4:16). So "Nod" just means "out from the presence of the Lord" (Genesis 4:16; whose presence remained in the Garden of Eden on earth, behind the 2 Cherubims, which was where Adam/Eve and Cain and Abel would come to (at the gates of Eden, like the sanctiury) to offer their sacrifices, etc).
The earth was populated by then.
The scripture does not say that at all. Time actually passes in scripture to when Cain "went out from the presence of the LORD", then "dwelt" in the land of "nod", and marries a sister (daughter of Adam/Eve, Genesis 5:4) and has a child, then "builded a city" (Genesis 4:17) to give it as an inheritance (on earth, earthly inheritance) to his own son.
Please take the time to prayerfully read the texts provided, thank you.