"Backtracked", a short story by Burt K. Filer, first published in The Magazine of Fantasy and Science Fiction, June 1968, available at the Internet Archive. It has been reprinted a few times; does any of these covers ring a bell? If you read it in the '80s, it might have bean in the 1979 anthology Beyond Reality edited by Terry Carr.
The story opens with the male protagonist waking up in the morning and realizing that his body has changed - his upper body is significantly stronger and maybe his skin is tanned?
The first thing he saw was Sally staring at him. She was sitting up in the big bed and had four fingers of her left hand wedged in her mouth. For some reason she'd drawn the sheet up around her and held it there with the other arm, as if caught suddenly by a stranger. Fletcher sat up.
"What's the matter? What time is it?" He felt odd and a little woozy. His voice sounded rough and both legs hurt, the good one and the other one.
"You've backtracked," Sally said. She gritted her teeth and gave that quick double shake of hers. The long brown hair fell down, and a curler came out.
Fletcher looked down at the arm he'd hooked around his good knee. It was sunburned and freckled the way August usually made it, but the August of what future year had done this? The fingers were blunter, the nails badly bitten, and the arm itself was thicker by half than the one he'd gone to bed with.
In this story, time travel allows you to return physically to a previous time, but without any memory of what happens next . . .
Sally lay back down, blinking, on the verge of tears. "You're older," she said, "a lot older. Why'd you do it?"
Fletcher tossed off the sheet and swung his legs to the floor. "I don't know, but then I wouldn't. It wipes you out completely, they say."
It turns out that the moment is the death of his wife, or what would be her death: they have a house that overlooks a valley or canyon or something, and she does something that causes her to fall over a railing or down an incline where she's holding on for her life.
Sally was on her feet again, picking more flowers. She paused from time to time to gaze out over the river. The view was even more spectacular here, Fletcher knew, even though too far back to see it himself. They were three or four hundred feet straight above the water.
Running a few feet beyond the main line of the bluff was a grassy promontory. Several bunches of Queen Anne's lace waved above the wild hay and creepers. He wished she'd get away from there and took a breath to tell her to.
Sally screamed as her legs slid out of sight. Twisting midair, she clutched two frantic handfuls of turf.
The protagonist realizes that past-him wasn't strong enough to save her and that future-him had spent all his remaining time bulking up to come back and do it. He is able to reach her (I don't think he simply wheels over to the edge, I think he has to leave his chair and drag himself to her, then pull her to safety) and she lives.
She was only sixty feet away, but the fireplace and the big old table lay directly between them. Fletcher planted both hands on the smoking stone chimney and vaulted it. The thing was four feet high, but it could had been five and he'd still have made it. A dozen running steps, each faster and longer than the last, carried him to the table. He yanked his head down and his right leg up to hurdle it, snapping the leg down on the other side and swinging the weaker one behind. Pain shot through it, and Fletcher nearly sprawled. It took him four steps to straighten out, and in four more he was there.
He hurled himself at the two slender wrists that were falling away, and got one.
Sally screamed again, this time in pain. Fletcher hauled her up to his chin, both sinewy hands around her small white one. Edging backward on his knees, he drew her fully up. Fletcher stood shakily and attempted to help her to her feet. His left leg gave way.
Falling beside her, he lay on the warm granite and tried to catch his breath. It was difficult for some reason. Her face swam before him, and as he lost consciousness he heard himself repeating, "So that's why, that's why—"
The twist is that while she survives, their marriage doesn't - she ends up divorcing him.
His broken left leg never healed solidly, though, and for all intents and purposes he was back to where he'd started. He and Sally remained childless right up until their divorce two years later. She was later married to David Schenk, but Fletcher remained alone.
