You see, this treadmill is cosmic-ray powered! 
The Flash #125 is an issue of the series The Flash (Volume 1) with a cover date of December, 1961.
Synopsis for "The Conquerors of Time!"
While nuclear-powered devices fail across the globe, Wally West arrives in Central City for a brief vacation. He is picked up at the train station by Barry Allen, who takes him back to his lab and presents his new Cosmic Treadmill. He explains that he designed it so he could travel through time. He then tells Wally of his travels to the year 2287. While he was there, he discovered that Earth had been invaded by the Dokris. He learns that they have extracted all of the radiation in the minerals of the planet by deploying a hive into the distant past. Due to this, the future race will have no atomic weapons in order to combat any alien invaders.
Barry sends Wally back to the year 100,842,246 BC to destroy the hive, while he travels to the future to help the people fight off the Dokris.
In the past, Wally befriends a strange birdlike creature after saving it from a giant ant. They in turn save him from a giant bee that carried him into the sky. He communicates with them by building a replica of the hive he is seeking out of rocks, and they soon begin to search for it. However, Wally passes out, paralyzed by the sting from the giant bee.
In the future, Barry finds a pocket of humans hiding in some caverns and convinces them to hold onto their atomic weapons, as they should soon work again. He discovers that an army of Dokris soldiers are heading to the caverns, but he diverts to another location to seal the advancing invaders underground. Unfortunately, one of the soldiers manages to escape his trap and shoots Barry with a radiation gun. Although he is able to sidestep the brunt of the blast, Barry is still weakened and is captured.
The Dokris are impressed with Barry's powers and offer him a chance to join them. When he refuses, he is imprisoned and constantly irradiated to negate his powers. Unknown to his captors, Barry has been carrying one of the era's atomic weapons, which the aliens did not take, thinking it was harmless. On his first attempt, it failed to fire, but on a second try it works, and he uses it to blow a hole in the side of his cell and escapes. An army of aliens are sent after him, but Barry defeats them by creating a storm of wind that bowls them over. With their atomic weapons working again, the humans are easily able to drive the Dokris away from Earth.
Upon returning to his home era, Barry listens to Wally's side of the story. The Ornitho-Men had cured his paralysis using leaves, and after they found the hive, it was easily destroyed.
Appearing in "The Conquerors of Time!"
Featured Characters:
Supporting Characters:
- Iris West
- Ornitho-Men (First appearance) (Single appearance)
Antagonists:
- Gigantic Ant
- Gigantic Bee
- Dokris (First appearance) (Single appearance)
- Mynher (First appearance) (Single appearance)
- Nekaf (First appearance) (Single appearance)
Locations:
- 2287
- Central City
- Atom-Powered Electric Plant
- Central City Police Department
- U.S. Nuclear Submarine
- Colossal Caverns (First appearance) (Single appearance)
Items:
- Cosmic Treadmill (First appearance)
- Dokris Hive (First appearance) (Single appearance)
- Mind-Probe
- R-Device (First appearance) (Single appearance)
Notes
- This issue is reprinted in The Greatest Flash Stories Ever Told, The Flash Archives Vol. 4, Showcase Presents: The Flash Vol. 2, DC Comics Presents: The Flash Vol 2 1, The Flash Omnibus Vol. 1 and The Flash: A Celebration of 75 Years.