I suppose I would find life tedious if I was to spend an eternity in the world as it exists today. But if there were always new worlds to explore, and new things to discover and learn? I could enjoy life quite a long time under those circumstances. I have no idea when I might grow tired of it.
It's boredom (and the energy drain of old age) that makes eternal life seem unpalatable. Could we avoid running out of excitement over a long enough timeline? I guess not. I could stand to do with more than the 70, 80, or 90 years I am likely to exist when all is said and done. But, as pete said, once it's over you won't be around to worry about it. So I'd rather enjoy the time that I have and not worry about the eternity to follow without me.