In man, the MLB usually is assigned the HB; in in zone, a middle hook zone. While a completion to the running back or to anyone in midfield but not deep can result in a 3rd-and-very-long 1st down conversion, I'd say it more often doesn't, especially when playing Cover 2 and protecting the sticks. I play mostly Cover 2 Man in 3rd and very long, and even if the MLB is late covering the RB, it's usually takes a really shifty, fast back to get the first down.
To me, 3rd and
very long is more than 10 yards, closer at minimum to 15 yards, usually the result of a sack on 1st or 2nd down. The guys I rely on in this situation are the DEs, the CBs, and the safeties.