Ultra Long Hauler, I like your attitude towards mountain flying. Although I want to point out that you really can't train yourself by flying a few times at 3000m then 4000m and so on. Similar to high altitude training of athletics you would need to stay at this altitude for some weeks. Then, your lung would adapt to air cointaining little oxygen. Also I strongly recommend to have another pilot with you. I believe you would not realize the lack of oxygen until it's too late.
I live in the Swiss alps above 5000 feet, I'm young, don't smoke. When flying e.g. to Italy over the alps at up to 12500ft I don't feel any changes in concentration. Still I would fly according to the FARs.