I inherited a Ruger Mark I from a great uncle and it's pretty old, I think. It's in the 61500 serial number range. It's just a plan model with plastic grips.

I don't know how to grade condition of a firearm, but it seems to be in good mechanical condition, I have shot it and it runs just fine. The bluing is worn in places from a holster and it's got some of that brown rust in grip area. Best I can describe it is there is a guy on youtube that goes by Anvil or something and calls it mung and boils it to "conserve" it. That's the best way I can describe it.

I don't expect it's got any real value, I would like to keep shooting it. I guess on of my concerns is parts availability if something did wear out or break.

I would shoot and enjoy it, those Rugers are built to take a lot of use.

Even if something breaks it is highly likely it could be repaired, it is not like it is an obscure brand/model.

I don't think so. All of my guns get some range time, including the C&Rs. This way I'm not ending up cleaning guns that haven't been shot :-) I also have common spare parts like springs,firing pins, extractors and stuff in my stash.
In particular with the ammo shortage it's a cool thing to have multiple calibers and multiple options to ensure the range days stay interesting.

This.