Tactics

Run and gun is about one thing - traversing the most often-traveled areas of a map quickly (hot zones), to encounter and ideally dispatch as many targets as possible. If this is not your style (it is most people's), you best course of action is to play directly against this behavior - effectively, play anti-run and gun.

The first key element to anti-R&G that you'll have to worry about (and really this is an advantage all the time, but with R&Gers speed it's even more relevant) - you don't want them to know where you are. If an R&Ger's dot/indicator shows up on a minimap, it's probably no big deal to them - they're already dead, or running away from that spot. You, however - you're a prime target for all the R&Gers in the area, instantly. Another key point here is that camping is probably a bad idea - you might get one kill, but there's a good chance that player is going to come looking for you.

The second key element is knowing where they are (or will be), and using that to your advantage. Getting to know the maps very well is incredibly important - specifically, the spots that are less often traveled, have good cover, where you can't be easily seen by someone moving quickly, or that allow good line of sight to major walkways (without being in a major walkway themselves) are the best. You don't want to stay in these spots long, however - after you get a kill or two, head in a safe direction under cover to another good spot.

Equipment

Weapon Proficiency: Silencer Obviously you'll need to select a weapon that can get a silencer. The key here, obviously, is that since you won't be able to move quickly away from a location you fired at, you don't want the shot(s) to show up on the minimap.

Perk: Assassin A similar circumstance - UAVs are an R&Gers best friend, any way you can keep off their minimap is a good way to play. Consider Blind Eye as well - R&Gers often get some nasty killstreak rewards if the rest of your team isn't prepared.

Lethal Equipment: Claymore, Bouncing Betties Once you know the hot zones, this is where you get to have some fun - if there's a good bet an R&Ger is going to come around a particular corner, make them regret it. You can also use these defensively - holing up to snipe with one of these at the entrance is always fun (though many of the good sniping areas have multiple entrances in MW3).