The Man Who Mastered Time

From the first page of the Ace Double: "A classic adventure in the far future - It took what seemed but half a day's traveling to traverse the 28,000 years that separated Loto Rogers from the most beautiful girl he had ever seen. He had expected to find mighty cities and a flowering civilization in that future world, but instead he found only ice and snow - and Azeela. Thought Loto had intended only to visit, he saw then that he alone could help the survivors of the centuries' incredible savages. And so The Man Who Mastered Time faced the one problem that his super-science could not answer - the mastery of the world's final show down."