Can I trust it? It runs smooth and is clean. Got it from an heating and colling professional.
I know its from the 90's but its not years but the mileage--correct?

Here's your owners manual

http://www.lennox.com/pdfs/manuals/Lennox%20G12%20Manual.pdf (http://www.lennox.com/pdfs/manuals/Lennox%20G12%20Manual.pdf)

All furnaces will run until they fail. Hopefully you will get many years out of it.