"You have to get out there, run around, get hit, run back to the huddle and run a play again," Dunn said. "I can usually go a whole series or a drive and really not get tired, but right now I can't run any plays without being exhausted."
Dunn, whose 9,461 career yards rank third among active players, will try to return for the Falcons' third preseason game when Cincinnati visits the Georgia Dome on Aug. 27.
A three-time Pro Bowl selection and the 12th overall draft pick for Tampa Bay in 1997, Dunn surpassed 1,000 yards last year as Atlanta led the NFL in rushing for the third consecutive season.
Dunn, 32, had surgery to repair a herniated disc five days before training camp began on July 26. After spending most of last week running, Dunn convinced coach Bobby Petrino that he was ready to take some snaps in full pads.
"I did ask them to get out there at the start of the week just to run some plays and get into football shape," Dunn said. "They're very cautious, and I understand, but I need to be out on the field. Right now, the most important thing is to think under pressure, react a little bit and get a feel before the season starts."
Barring a setback, Petrino sees no reason that Dunn won't be ready for the regular season opener at Minnesota on Sept. 9.
"When you watch him run, you get excited," Petrino said. "He was able to get in there today and begin his football conditioning because football is so much different than just running sprints or running strides. Overall, he looks good."
The Falcons will start Jerious Norwood at running back for the second consecutive game when Atlanta travels to Buffalo for a Friday night preseason game.
In other injury news, tight end Alge Crumpler, who's sidelined with a sore knee, also will try to make his preseason debut against the Bengals.