There is much evidence of many God's law being in place through Genesis prior to when they were given at Sinai. For example, murder was sinful back in Genesis 4:7 and Joseph knew adultery was sinful in Genesis 39:9. In Genesis 7:2, Noah had been given instructions about what to do with clean and unclean animals without being told how to tell them apart and in Genesis 8:20 he knew to offer a clean animal, so it can reasonably be inferred that he was given prior instructions in their regard.
There are a number of verses that describe the Mosaic Law as being instructions for how to walk in God's ways, such as Deuteronomy 10:12-12, Isaiah 2:2-3, Joshua 22:5, Psalms 103:7, and many others, so the Law was given as instructions for how to reflect God's attributes, such as holiness, righteousness, goodness, justice, mercy, faithfulness, love, joy, peace, patience, kindness, gentleness, and self-control. God's attributes are eternal and unchanging, so His instructions for how to reflect His attributes are likewise eternal and unchanging, which means that we can expect that what God taught to Abraham is very similar to what God taught to Moses. The main difference would be in regard to instructions for how Israel was to act as a nation and to the Levitical priesthood.
In Galatians 5:14, anyone who has ever loved their neighbor has fulfilled the entire Law, so there are countless people who have kept the Law. Disobedience to God's Law is not because it is too difficult for us to obey but because of the hardness of our hearts, which leads us to disobey it. This is why the New Covenant involves God taking away our hearts of stone, giving us hearts of flesh, sending His Spirit to lead us in obedience to His Law (Ezekiel 36:26-27), sending His son to free us from sin so that we can be free to obey that Law that we might meet its righteous requirement (Romans 8:4-7), and writing His Law in our hearts so that we will obey it (Jeremiah 31:34).