These words are often used interchangeably, but they have distinct meanings. Endless means "without an end," "boundless," "interminable," "continuous": "The endless prairie stretched before his eyes." "During his recent illness, he felt that his pain was endless." "The workmen are replacing an endless conveyor belt." Innumerable means "too many to count," "countless," "numberless": "The daisies in that huge field were innumerable." The tasks of a housewife may seem endless, but they are hardly innumerable. The leaves on a tree may be innumerable but they are not endless; neither are the innumerable snowflakes falling in a winter storm.
Dictionary of problem words and expressions. Harry Shaw. 1975.