1) Humanity existed more than one million years. G-d was discovered or invented by the Jews 4000 years ago. Why did it take him more than one million years to announce that He is the creator?
2) Adam and Eve had two children, Cain and Hevel. How did they practice "pru urevu"?
1) The fact stated in your question regarding the existence of the world for over a million years is indeed a theory of scientists which seems to supposedly be contradicting the Torah. This is not the right platform to discuss this issue which many have already discussed and written many articles and books to resolve this matter. I will write briefly two of the approaches on the issue. The scientific research calculation of the Age of the world is based on the premise that the world began its existence and its long way of development from 0. This premise is completely incorrect. Our sages say that "All the creations of the Genesis were created in their full-grown stature" (Numbers Rabbah 12, 8. Talmud Tractate Rosh Hashanah 11a, Chulin 60a, and Rashi Genesis 1, 25). There is another approach that determines that the first days of creation are not days of twenty-four hours, but a description of time periods that lasted millions of years.
See the article “The Age of the Universe” by Dr. Gerald Schroeder:
See also the book “In the Beginning: Biblical Creation and Science” by Professor Nathan Aviezer.
Regarding your question; it did not take The Blessed Be He time to announce His existence. G-d turned and spoke to Adam and Eve immediately upon their creation as it says: “And God created man in His image; in the image of God He created him; male and female He created them. And God blessed them, and God said to them, Be fruitful and multiply and fill the earth and subdue it, and rule over the fish of the sea and over the fowl of the sky and over all the beasts that tread upon the earth” (Genesis 1, 27-28). Beyond that; the very act of creation is the biggest announcement of the Creator’s existence. But the man corrupted his way upon the earth, and throughout the generations more and more people turned to idolatry, until our forefather Abraham came and recognized The Blessed Be He, and worked to bring back humanity to faith and knowledge of the Creator of the world. G-d therefore viewed him as the ideal person to announce G-d’s word in the world; therefore his children the people of Israel were chosen to receive the Torah at Mount Sinai and to be the nation of G-d that bears the great role of being a light unto the nations and to publicize the great name of The Blessed Be He.
2) Adam and Eve had a few more children.
Rashi in Genesis 4, 1 quotes the Midrash: Cain… his brother Abel: In Hebrew; Et Cain Et Achiv Et Hevel. The word Et is repeated three times to suggest additional things. This teaches that a twin sister was born with Cain, and with Abel were born two. Therefore, it is said: Vatosef, and she continued, or added. — [from Gen. Rabbah 22:2, 3].
In Genesis 4, 25 it says: And Adam knew his wife again, and she bore a son, and she named him Seth, for God has given me other seed, instead of Abel, for Cain slew him.
And in Genesis 5, 4 it says: And the days of Adam after he begot Seth were eight hundred years, and he begot sons and daughters.