You do not have to be good at sports to practise Zhineng Qigong. It may be even detrimental to have done sports before.
During sports you train muscles and work with muscle power. You use energy and power and after the physical exertion you are tired or even exhausted.
With Zhineng Qigong you learn to leave out the muscle power. The movement is done with the mind and with qi. The muscles are relaxed so that qi can flow unimpededly. After the practice you have gathered qi and feel fresh and motivated.
The fitness that you can achieve by regular practice has nothing to do with muscle training. The shape of the body gets more firm and elastic. The agility increases. The movements get easy and effortless.
Done with qi, you can repeat a wallsquat 100 times. Done with muscle power, after a very short time you are exhausted.