Courtney Brown is talking about 3 hours maximum, if you include the exercise. Many people spend more time than that each day in traffic or on a train!
If you double your life span, then it would be worthwhile 'time investment' anyway and you will likely feel better and enjoy life more as a result too.

Does attempting to extend life span really lead to more fulfillment or simply stop you living happily in the now because youre too busy contemplating what may or may not happen in the future ?

I don't see why life extension contradicts living in the now. What is stopping you from doing what is needed to extend your life and remaining in the now moment? Both of these guys specifically meditate.

Dont you think natural lifespans are natures way of maintaining balance ?

Death is not natural or balanced and in fact, death is the result of imbalance.

If humans doubled their life expectancy, how would the planet cope with the additional resource requirements ?

Many families in poor areas have many children specifically because they know that some will die. The more stable our life is, the less pressure we have to create more people. We are not entirely stupid creatures and will adapt as our environment changes. We can stop our robotic programming. ;)

Without death, life doesnt exist by definition

I think then that you have the wrong definition for life ;)