Very recently I got a fit bit and it has to ability to calculate a resting heart rate for the day, over a long period of time, could this be a used as a measurement for fitness or conditioning for a sport?
It could as long as it’s accurate. This article talks about heart rate variability training and how to let your heart rate influence your programming.
Another simple way is to monitor resting heart rate first thing in the morning, right after waking. Establish a baseline and let any changes to your “normal” dictate your plan.