Oddly enough we have an idea on what people ate in time of Ivan the Terrible, or roughly 500 years ago. I don't really know of sources pertaining to anything prior to that, however I have an invaluable resource when it comes to the time of Ivan the Terrible. There was a book called the Domostroi which was a lists of "Rules for the Household" in Russia. In it there was recipes, and "proper" ways to feed your servants and serfs. There is actually an English translation by Carolyn Johnston Pouncy and can be picked up on your favorite bookseller or library. However, here is a few snippets on what to feed your servants!

"As everyday food servants receive rye bread, cabbage soup, and thin kasha with ham. Sometimes they may have thick kasha with lard. This is what most people give their servants for dinner, although they vary the menu according to which meat is available. On Sundays and holy days servants sometimes get turnovers [pirozhki—small pies stuffed with a mixture of meat or vegetables, cooked grains, and eggs], jellies, pancakes, or other, similar food. At supper they eat cabbage soup and milk or kasha."

Some other thoughts:

Kasha seems to be a recurring theme in Russian cuisine and early Russian cuisine as well. There also seems to be a focus on fish as well. However this likely could be very dependent on the location of your home. Cabbage is a staple, for sure. I did find something, although it is far more contemporary, it probably does have older roots, and that is Mushrooms. Mushrooms kind of make a recurring theme at some points, even spawning a quote from Vladimir Nabakov in the 20th century when he said the following:

"One of her greatest pleasures in summer was the very Russian sport of hodit’ po gribi (looking for mushrooms). Fried in butter and thickened with sour cream her delicious finds appeared regularly on the dinner table. Not that the gustatory moment mattered much. Her main delight was in the quest."

Hopefully this helps, in your quest for old Russian food!