Guys I know this is probably inaccurate in so many ways but I was listening to the soundtrack for the TV series and this idea came to my head and I had to write it! It's a mix of everything I've read and watched about the Flash...
Barry was old and he chuckled to himself as he walked on to the worn out treadmill he had used since becoming 'The Flash'. To his wife, Iris, his walk was a jog. Despite his age he could still be a speedster, albeit slower. But once the coughing took over and he doubled over in a fit to catch breath Barry new The Flash was dying within him. He knew it, Iris knew it, his family and friends knew it. The world knew it.
Everything seemed so slow, so depressingly slow. To Barry, it was as if the world was slowing to a halt. Barry sat down in his chair, his bones had began aching as years of fighting and lack of regenerative healing took it's toll, he yearned to run, his suit hung in his room, untouched from his last escapade, covered in layers of dust.
He and Iris had a son and a daughter, both of which showed signs of speed. Their grandchildren also carried "The Flash gene" as Iris liked to call it. Barry would go to all of their school races and watch them, each time it would bring back old memories. He would help train his children and grandchildren, like Professor Wells, perhaps even better.
He knew it was time. Barry heaved himself from his chair and headed towards the door, leaning against the porch railings was the young Flash he had once been, dressed in his scarlet suit
"Hello old man" The voice echoed in his mind
"Slugger" Barry replied, smiling
"You ready?" The Flash asked
"As ever" Barry laughed. The Flash smiled as he pulled his mask over his head
"Where to this time?" The Flash asked
"Anywhere" And they both began to run. The world melted into a blur as streaks of lightning trailed behind them, both Barry and the Flash shared sideways glances
"Come on Flash, just one last race" Barry laughed as they both ran into the swirling light of the speed force.