Usagi Yojimbo DojoBoard

(NOT that Usagi is a manga. Though a lot of people seem to think it is... it's just, I think... well, you'll see.)
How are manga "planned"? (How are COMICS planned?) How much do you work out before you start writing the story? I hear from lots of authors (Stan Sakai, Watsuki Nobuhiro [rurouni kenshin], Yu Watase, and on and on for forever and ever!) that they "weren't originally intending it to be 'that long'"-- usually the author was only intending his/her series to go on for a few episodes, or a few volumes-- and then it goes on and on forever.
So my best guess is this: As comic book authors, we have to produce so much so quickly, that we can't put everything into one series-- because we're not sure (writer OR reader) that we'll enjoy it or that it will be a big success. So, what I'm guessing is that we plan a SHORTER story-- a test run, so to speak. And if that goes well, we probably do a second test, and then a third, and if that goes well, THEN we dedicate ourselves to it.
Is this how it works? If so, how do you avoid boxing yourself in?
Do you just do lots of short stories until one just grabs everyone? I'm lacking some direction here, and some advice would REALLY be appreciated.

Yo'; You've REALLY got to LOVE what you're doing. You've got to be comfortable with your subject matter and your characters. I'm sure that Sakai-sensei doesn't feel"boxed in" by Usagi because he's got so many stories in him to tell. So far...not a "clunker" in the 20 years he's been doing Usagi. Because he "knows" his characters intimately and once you get in the "zone" or "groove" you can keep going for as long as you want. So...stick to your guns and keep the faith dude! Ja mata...Abayo!

Usagi originally was created to be a secondary character in the Nilson Groundthumper story. Nilson's story was to be about 1,000 pages with a definite beginning, middle, and end. Usagi was enter the storyline in the middle.

However, the more I thought about Usagi, the more I liked him. Nilson and Hermy were put on the back burner, so to speak, and I concentrated on Usagi. I've now done about 2,500 Usagi pages.

Nilson and Hermy haven't appeared in years, though I still want to do a crossover story with Usagi.