The phrase "one's return to full fitness" means that someone has recovered completely from an injury or illness and is now as physically fit and healthy as they were before. Full definition
Even better, he recently claimed to have just returned to full fitness, so we should expect even more from him in the future. (latestchelseanews.com)
He will be hoping to return to full fitness soon before enjoying a run of games in the second half of the campaign. (squawka.com)
Instead, the club's hierarchy were forced to hold out hope that he could one day return to full fitness and be something more than a mere shadow of his former self. (chiesaditotti.com)