From: Iowan in MD/DC
One of the primary reasons the Japanese lost the Guadalcanal campaign was their "victory disease" - they continually underestimated the strength required to successfully push the Americans off the island, and so continually committed far too small of a force to accomplish their objective. When they finally started getting serious about it, Henderson Field (and Fighter Strip One) were continually operable and able to completely shut down any serious attempts to retake the island.
In any case, I'd like to add that if you know you'll be landing unopposed, you can get away with having low (or no) preparation for your target on an amphibious landing. But if you do land against opposition with little or no prep, you can expect massive disruption and disabling of squads/devices. I've found, in my newbie bumbling, that having CAs and BBs capable of suppressing shore batteries somewhat mitigates the disruption/disabling, but it's still pretty bad.
As for pushing through multiple hexes, if there is no base or dot base in the hex then planning makes no difference. Also, as I understand it, if the opposing forces are also not prepared for the base/dot base in question, then there is no positive or negative modifier for preparation for either side (vets, feel free to jump in here...). For example, if you were to make the allies retreat from Buna into the hex between Buna and Moresby, and then follow them there: since that hex has no base, prep doesn't matter for either side.
Example 2: the allies have Tassafaronga and Lunga but all of their forces are at and prepared for Tassafaronga. You invade Tassafaronga (taking it) and the allied units retreat to Lunga. You then pursue to Lunga, and all forces on both sides are prepared for Tassafaronga. In that case, no prep bonuses or penalties on either side. (It would be a good idea to begin prepping for Lunga, however, in case you are in for a long siege.)