Too big a kettle?

Sorry if this has been answered before, but like many of us I'm about to start my first batch and I just realized I have too big a kettle. I believe I have like a 12 gallon kettle, and my recipe (AHS Double Chocolate Stout) like many other recipes is telling me I should heat 2 gallons of water, however.. since my kettle is too big I'm not going to be able to steep my grains and I really have no other option other than 2 3 gallon kettles, and I think I'm going to end up with an overboil.

Is it possible for me to just put all of the 5 gallons in it and do the recipe this way? I realize it's going to take forever to boil and to cool, but I really see no other way.. Any changes to the recipe if I were to do this?

Also, is it possible for me to transfer all the 5 gallons on to my primary bucket and cool it there? Or would something happen to the beer?

I would greatly appreciate any feedback.