First you need to learn how to do a 0-death run (playing through the game without losing a single pikmin). Next you need to master it. Then, slowly reduce the amount of time taken to collect all fruits with no pikmin deaths. When you can do it in about 15 days or less, you may consider trying a 7-day run. Finally, once you can do that with a 0-death run, a 10-day run won't seem so impossible.