"It feels good to get out there and jell with the offense," Jennings said. "Find a big hole like that and make a big play."
Manning said the offense under new coordinator Ben McAdoo is a work in progress.
"I thought the running game was good," Manning said. "Whenever you have a running back break a long one, that's a great thing to have to be able to hit that big play.
"I feel confident in the offense, feel confident in our ability."
Jennings, who signed a four-year, $10 million deal with the Giants in March, finished with 85 yards to lead the team.
The seven-play, 70-yard drive ended with the first of Shaun Suisham's three field goals, a 26-yarder for a 3-0 lead.
"I thought it was a good, solid first series," Roethlisberger said. "Obviously, it helped that we had a huge play to Dri. But we moved the ball down the field and got points on the board."
Jennings' long touchdown run came on the next play from scrimmage.
"It was a well-executed play," Jennings said. "Eli made sure we were in the right play, it was a hat on a hat, a gaping hole in the offensive line."
"One-on-one battles in the open field, you've got to win those. It's what I did and the rest was just a race."
"We gave up a long run and that can't happen," he said. "You're going to have guys break out and get 8 or get 12 yards, but not 78. It was an integrity issue. When those things happen, you have to be able to minimize the explosion."
Suisham added a 32-yarder 59 seconds into the fourth quarter to make it 13-9.
NOTES: Washington also caught a TD pass last week against the Bills. ... Steelers second-year linebacker Jarvis Jones had a sack of Manning late in the first quarter. ... First-round pick WR Odell Beckham Jr. missed the game because a hamstring injury. Also out for the Giants: WR Trindon Holliday (hamstring), OT Will Beatty and DT Mike Patterson (shoulder). ... Missing the game for the Steelers: LB Ryan Shazier, TE Michael Palmer, WR Darrius Heyward-Bey and NT Steve McLendon. ... The Giants' next preseason game is next Saturday at the Colts, who lost to the Jets on Thursday. The Steelers will host the Bills in their preseason home opener next Saturday.
Copyright 2014 by The Associated Press