Seems to me you have two areas where you want to optimise; I've run large games before (8 players in a Rolemaster campaign, I must have been mad!) and keeping all the players involved is tough, here's what I learnt from it.

Combat

• Preparation; I don't know about AD&D 4e but I do know about complicated systems and preparation can save you loads of time. Make a carddeck of monster information you can quickly refer to, no referencing is quicker play.
• Numbering. Number everything and order them and write a short name after it for fluff. Get a bit of paper and write down the initiative order for everyone in a fight and list your monsters as numbers; then track resources next to their name. While a player is deciding you can decide what the next monster is doing.
• Tracking: Monsters can be very complicated to track with multiple resources, hit points, abilities and so on. So keep them simple, put check boxes in your initiative list to mark what they've used so you don't have to keep checking; if it helps. Also you can do the same for the players; make a table where you track each players hits, ac and so on so you don't have to keep asking if you've hit, if they're at 50% hits or whatever.
• Mooks and minions. It's been mentioned before and it's a great timesaver; Splat the 2 hit point Kobold really has only two states, annoying and dead. Don't bother counting.
• Simplify, simplify, simplify: Round monsters stats off, ignore time consuming abilities, if they're nearly dead make them really dead. If a monster is unconscious don't bother with stabilisation rolls (for example) fudge things away dramatically to save time, keeping the fight flowing is more important.
• Rounding: If maths isn't your forte round damage to the nearest five or ten either way and it'll save you a lot of headaches and counting; you can even then use a simple bar/gate system to add on damage in blocks of five to count damage.
• Poke players in order, tell the player who's next after whoever's deciding to get ready each time. "Bob you're up, Eric start thinking of what you want to do."
• If you really want to harsh it, give players a decision time frame; 1 minute per turn, maybe less. This will encourage them to prep their decisions!

Social

I notice in your question you have When we do story based adventure there are 2-3 players that do most of the talking and the others twiddle their thumbs. To me, social situations are where a GM can be the most lazy, especially with a big group.

• Encourage PC to PC interaction, backstories are gold for this; players should be talking to each other and deciding stuff as much as possible.
• Draw out the quiet ones, it's been said before but "victimise" those who keep quiet, encourage them, drop bits from background into their story "Bob you notice an old book on the shelf that is titled 'A history of Bobland' where your old village Bobford was" and so on.
• Round Robin it. Break the group into smaller ones and give each one a bit of time; Two groups are out buying stuff? Tell the first about the shop, do a bit of interaction then switch to the other group for a bit - this can take a bit of getting used to but I've found it very helpful in stopping players sitting around being bored - just switch every few sentences or so between groups, the other group gets a chance to think some more and the previously waiting group gets a go.
• By the same dint give NPCs in social situations more people. How intimidating is it for 8 people to try and talk to the same person at the same time? Add in the Wizards familiar to talk to, another shop assistant, add in extra NPC's to situations so you can use the Round Robin tactic.

It will take some work, but it's perfectly doable.

Enemies with special defenses

The players won't have access to see invisible, flight, magic weapons, or ghost touch. Therefore, if you include enemies that can fly, or have DR/magic, or are intangible, these will be huge challenges, and may be downright impossible. The easy solution is simply to not include such enemies, but the more interesting way is to treat them as nigh-invulnerable enemies that the heroes will have to figure out how to defeat. In a normal Pathfinder game, a ghost is a normal critter with a +2 CR template pasted on top, and requires some minimal preparation to defeat. In a low-magic Pathfinder game, a ghost is a mystery: why did they become a ghost? How can you persuade them to pass on, or at least let the party pass by peacefully? Can you persuade the local priest to perform an exorcism, and will it even work? Instead of "find monster, insert fireball," these types of encounters are now role-playing challenges, because they can't be solved any other way!

Alternatively, you can simply strip out the special defenses from enemies. Pathfinder assumes you have level-appropriate counters to special abilities anyway, so by removing those special defenses, you aren't going too far from the original intent. Adjusting the CR is left as an exercise for the GM, because it's going to take a fair amount of trial-and-error to determine what the right balance is.

Fixing armor class, and other numbers issues

Pathfinder assumes that both attack bonuses and defenses will be augmented by magic items. This partially balances out if the players don't have magic items, but consider giving everyone a +1 bonus to AC and all defenses every four levels. Don't make them pay a feat for it, just give it to them.

While you're at it, give your players bonus XP for the monsters they defeat, by calculating the XP as if the monsters were a higher CR. Since they're operating without magic, every encounter is going to be harder than what the DMG "expects" when it calculates XP per CR.

What will they do with their money?

Your players won't be able to buy gear that personally enhances their ability to make things dead faster, or grant them new solutions. If you keep to the normal loot rules, then the party will have far, far more money than they know what to do with. You have two options here: give them less money, or give them something to do with that money.

Let them invest in mercenary companies or land holdings. Let them become influential in the church, or their hometown, or even their country as their economic might and donations in the right places give them power that they would never be able to take with a sword. Favors in high places give characters some very powerful options.

Recovery after combat

This will require explicit house rules; you'll need to accelerate natural healing (heal a percentage of HP per day instead of a flat amount?), allow Heal checks to do much more than they normally do, grant the local clergy some extremely localized healing powers (they can heal people brought to their church, but not outside of their place of worship), and/or make this a political game rather than a hack-and-slash game.

If everyone's having fun, then it's a good game. It doesn't matter if the characters aren't optimized: as long as they feel like they're making a difference in the world and they're enjoying the game, then you're doing it right. The characters will be balanced, more or less: they all don't have access to magic, so intra-party balance isn't as much of a problem. You'll see that the players lack all of the magic-based solutions that you'd expect in a normal Pathfinder game, and you'll select the enemies more carefully, but things will work out fine. Let your players know that things will be a bit different than normal, and your players will go along with it; they requested this kind of game, after all.