Take a look at this excellent fitness video with Flight Engineer Jack Fischer; I'll bet he runs further than you do.
(Long Distance Runs by astronauts)
It turns out that the Combined Operational Load Bearing External Resistance Treadmill (COLBERT) was predicted long ago by thoughtful science fiction writers.
In his 1953 novel Space Tug, Golden Age science fiction master Murray Leinster writes about a gravity-simulator harness:
"When we got back," Joe told Brown, "we were practically invalids. No exercise up here. This time we've brought some harness to wear. We've some for you, too..."
Joe got out the gravity-simulator harnesses. He showed Brent how they worked. Brown hadn't official instructions to order their use, but Joe put one on himself, set for full Earth-gravity simulation.
He couldn't imitate actual gravity, of course. Only the effect of gravity on one's muscles. There were springs and elastic webbing pulling one's shoulders and feet together, so that it was as much effort to stand extended—with one's legs straight out—as to stand upright on Earth. Joe felt better with a pull on his body.
(Read more about Leinster's gravity-simulator harness)