Why were my banana muffins hard?

In the given recipe, assuming generously that the bananas came out to 1 3/4 c, the total water is:

Approximately 12 oz from banana
Approximately 3 oz from eggs

With three cups of flour, assuming a 4.5 oz cup of flour, this is a ratio of 0.9 : 1 flour to water by weight.

Compare to a similar banana muffin recipe (this one from the Food Network):

2 1/2 cups unbleached all-purpose flour
1/2 teaspoon baking soda
3/4 cup dark brown sugar
1/4 teaspoon ground cinnamon
2 cups smashed bananas (about 4 to 6 bananas)
1/2 cup vegetable oil
1/2 cup milk
2 large eggs, at room temperature
1/8 teaspoon fine salt
1/2 teaspoon pure vanilla extract
3/4 cups chopped walnuts

The water content of this recipe is approximately:

 4 oz from milk
 2 oz from eggs
 14 oz from banana

Again, with a 4.5 oz cup of flour, the flour to water ratio is 0.56 : 1 flour to water by weight.

Note that the recipe used has a much higher flour : water ratio, making a much drier and stiffer batter. It sounds like nearly a dough. It also has a relatively huge amount of sugar and oil.

My guess is the original recipe was not well designed and tested. It is highly likely that:

• The recipe you have used simply has a very low hydration of the flour, which would contribute to poor texture
• The low hydration encouraged overmixing, which developed gluten and created toughness

You have not indicated time or temperature or muffin size, but it is also likely that they were overbaked.

I would suggest finding a quality recipe as a base. Also, chocolate chips are great in banana muffins, but that is a separate issue.