to get back in shape
to begin exercising in order to become fit and healthy again
to be under the weather
to be sick
to be full of beans
to be active, lively, healthy, and have a lot of energy and enthusiasm
To be as fit as a fiddle
To be in an excellent state of health and physical condition
to be as strong as an ox
to possess great physical strength
to be run down
to be very tired and unwell due to overwork
to be up and about
to be out of bed after having recovered from an illness.
to be fresh as a daisy
to be well rested, awake, and energised