Use 'fighting fit' to stress excellent health, not just being okay or slightly well. After overcoming the flu, she's now fighting fit.
Pro Tip 2/3
Physical Vitality
Imply physical vigor, suggesting someone could handle demanding physical tasks. He trained hard and is now fighting fit for the physical challenges ahead.
Pro Tip 3/3
Recovery Indicator
Indicate recovery from illness or inactivity, returning to a strong, vigorous condition. After her knee surgery, she's now fighting fit and back to jogging.