Does 6,000 years feel like a long time to Adam in eternity?
No. Eternity is outside time, so time is not relevant. Even though Adam has been dead 6,000 years, when the redemption of the body takes place he will think only a brief moment has elapsed and will not realize 6,000 years passed. Earthly chronology will not govern his experience.
Les explains that when we go from time to eternity, it is a whole different set of circumstances. Because eternity is out of time and time is not relevant, even though Adam has been dead 6,000 years, when all this takes place Adam will think only that much time had elapsed; he will not realize that 6,000 years have gone by. As Les says, there have not been that many years so far as they are concerned. The context also stresses that the body has not yet experienced redemption and is still under the curse, so we get sick, get wounded, and, unless the Lord comes, die.