relal km/mileage in my bike

[trionyx]

Hello,
I'm quiet new in this forum..a few weeks ago I have bought a bike which is built in 2003.
After a few weeks as I get more familiar in this bike - I started to read this forum - I noticed that the chain is too long....and it has also been already changed, it is a Regina O-ring chain mounted. As I know the original chain is made by DID. So I started to think about the real mileage of the bike.
I mean the instrument displays 7000km (approx. 4300miles), but the chain isn't the oroginal one, and it is also weared. The sprocket and the rear toothed wheel is in perfect condition (no sign of any wear, still the original ones.)
Also the tires are the original, already 9 years old, the DOT number is the same as my bike manufacturing year. Each tire is Sava (original tire in the European market) The front tire looks great, thread depth is more than 4mm (not measured exactly), the rear tire is already more used, it's thread is 2,6mm deep.
What is your opinion, how long the original tire lasts, how many km/mile colud be the real mileage of this bike? What do you recommend to check, which parts does really showes me it's mileage?
Thanks in advence!