I am going to throw a short common sense theory out there that is even hard for me to comprehend. If you are feeling run down, tired, sick, or joints are aching it is ok (even better for you) to skip a training session!
Sometimes instead of working harder you can actually get stronger, fitter, and closer to your goals by recovering. I measure my recovery with a tool called the Morpheus Recovery Band, through a small test that takes into account my hrv, previous day training session, and sleep gives me a recovery value for the day. If my score is low I know recovery techniques are called for that day. The good news is you don't really need this tool to judge when recovery days should happen, just listen to your body. If you don't have it that day switch to a proven recovery technique and train hard tomorrow.
Here is a list of my go to recovery techniques:
Epsom Salt Bath
Mobility Training Session
Very Light Short Run or other Conditioning Session
So in closing, listen to your body and don't be afraid to skip a training session in favor of recovery. Remember forcing yourself through a hard training session when you don't have it can move you farther from your goals and not closer.