When you picture getting started straight away the close temporal distance puts you in near mode, where you see all the detailed impediments to doing a perfect job. When you think of doing the task in the future some time, trade-offs and barriers vanish and the glorious final goal becomes more vivid. So it always seems like you will do a great job in the future, whereas right now progress is depressingly slow and complicated.
This set of thoughts reminds me of those generally present when I consider the likely outcomes of getting further qualifications vs. employment, and of giving my altruistically intended savings to the best cause I can find now vs. accruing interest and spending them later. In general the effect could apply to any question of how long to prepare for something before you go out and do it. Do procrastinators invest more?