Drop rates are random, those numbers are just an average. It doesn't matter if you do it all at once, or at various times. The crawdad has an additional factor, you need to get Mr Pinchy, and he'll give you 3 "wishes" over 3 days, you might get the Magical Crawdad but you might not (with one outcome you actually get attacked!) If Mr Pinchy doesn't give you a crawdad and you've used up the wishes, you need to fish for another Mr Pinchy and try again (it isn't as hard to get the crawdad as it used to be, though).
This page at El's Extreme Angling has more tips (this website has info on every fishing goal in the game) http://www.elsanglin.com/catching_furious_crawdad.html
On one of my characters I racked up stacks and stacks of Furious Crawdads but never did get Mr Pinchy, on another I fished only a few pools, got Mr Pinchy, and got the crawdad the next day (my second wish). A lot of it is just luck, but you should max out your fishing as much as possible. Use the Weather Beaten Fishing Hat (do fishing dailies to try to get it if you don't have it, you can also get a better fishing pole that way or from the Kalu'ak), enchant a spare pair of gloves with +5 fishing if you can, and most importantly use the highest lure you can get your hands on, the +150 "Heat-Treated Spinning Lure" if you can get it, otherwise use one of the +100 lures. El's Extreme Angling has a page on fishing items, again, basically any kind of fishing info you'd want there. Good luck!