Well, for starters, I'd say don't use D&D. It is a game tailored towards violent conflicts, which is exactly what you're avoiding, it seems. Mind you, I said "violent conflicts". No story, thus no game, can exist without any conflict whatsoever. I'm not also saying it's completely undoable with D&D, just mainly... a waste of its design and practical goals. Another way to put it, to use a metaphor, is: smartphones are great, you can do a lot with them, they're like handheld computers... But they can't really substitute a desktop computer in every way, maybe not even most ways.
Now, if you're willing to work outside of D&D, there are some good systems out there for that "action and adventure doesn't mean swinging swords all the time" vibe you're after, like, for example, Fate (The Dresden Files RPG, Spirit of the Century, Diaspora, etc), in which most of the mechanics about resolving conflicts are the same, regardless if it's a brawl, a wardrobe and style show off or even an economic dispute between Lex Luthor and Bruce Wayne. It's worth a look, really, and there are SRDs available for some of those games (Spirit of the Century and Diaspora, iirc).
If you're sticking to D&D, plotting the campaign isn't the difficult part; the difficult part is to design encounters (which is just a way of saying "conflict scene") that allow your PCs to shine doing their thing when most of their sheets are geared towards combat (yeah, players do that, it being the game it is).
For the wizard (most hocus-pocus folks, really) and rogue, that's easy. Most other types, though, will rely purely on RP, most of the time, which isn't bad per se, just kinda unfair, since some players get to look at their sheets and say "I can do this, this and that", while the fighter's player has to memorize lines from Gladiator and the ranger's player has to become a living Bear Grylls encyclopedia.
If your group can pull it off, that campaign'd be the stuff of legends, but it'll be hard, really.
Quickly, I thought that my DM wrote this question, because we were in that exact same scenario. What helped us was:
The experienced players knew the material, and each took a pupil to help guide the new players. We're still having trouble actually "roleplaying" as a group. Most still treat it as a series of battles and not as an interactive story. We're getting better as a group, but be prepared for inexperienced players to decide that all they want to do is kill things. An overwhelming battle is a good idea for this. Make them need to retreat/consider not engaging.
Our DM used the total number of players as the guideline for us. Instead of 6 orcs, there were four orcs and two ogres. Instead of a group of skeletons, there were two wights and a high level mummy. Some caused problems, some didn't, but it was all fun.
Never underestimate the power of deus ex machina. An inexperienced DM can overwhelm his party. He can also use deus ex machina to solve the problem he created. For instance, we are a group of 6 (2 experienced players), and we had an encounter suitable for 8 characters. When it was clear we were in trouble (it was our 3rd/4th encounter), the DM used his god-like powers to help us out. We were near a town, and townspeople "witnessed our heroics and were inspired to help us" and took some flanking positions near some of the weaker enemies. This allowed our healer to retreat and heal the two tanks, and the rogue and ninja to clear out using sneak attacks. I think one villagers did 2 damage with a pitchfork. The damage wasn't the important part, but the attacks weren't centered on as many of the party, and the flanking helped with extra damage/increased attack rolls.
Just remember to think of creative solutions to problems that you will create, even just by mistake. In a dungeon, perhaps a trap springs and hits an enemy, rendering him unconscious for 5 rounds. Thing like this will allow the newbies to learn combat, and you can phase out the deus ex as you learn to better estimate your players skills and knowledge of the game.
Best Answer
Don't. Split the party into two groups.
I've been gaming 25 years and have played in home groups, at cons, and at gaming clubs. Without any exception, in my experience games that large are terrible. You can try "keep people on track" techniques, but in the end you're putting lipstick on a pig. No one will be able to do satisfying roleplaying, encounters will be difficult to balance, social problems will emerge from the loud folks causing chaos and the wallflowers being overlooked more than normal, etc. Heck, even parking on the street and bathroom availability becomes an issue.
In our gaming group, whenever we get too many folks interested in one game, we just split it. We've had several times where one GM is running two parallel sessions of a given Adventure Path, for example. Or two people agree to run it.
Your players that want more fun per hour of time spent will just drop out anyway, either to not play or to set up a second game, in which case you get the same result but with the bonus of potentially some hard feelings.
Two five-person groups will get way more done than one ten-person group and have more fun doing it. If you can't spend 2 nights a week on GMing, then alternate them biweekly.