Some of the heavy communities have full-time mission planners, the "desk jobs" of both pilots and navs. I was a mission planner for the U-2, which is a job generally staffed by recce Navs, but there were a handful of pilots in the shop, too.
In my experience in the fighter and trainer communities, most of the things the civilian world would consider "flight planning" were canned profiles that many times didn't even get updated with that day's weather and winds. What *was* more intricately planned were the tactical portions of the flight.
So, notional airspace boundaries and locations. Threat and target laydowns. Altitude block allocations for formation deconfliction. Radio freqs, IFF squawks, radar channels. Joker and bingo fuels for various times/locations/scenarios. Weapon attack profiles. Navigation routes, system updates, ground attack targeting plans. Post-target rejoin and egress plans. Production of briefing materials and in-cockpit reference materials for all of the aforementioned stuff.
For most missions some kind of computer-based software was used to produce nav points and reference data for the aircraft. Often production of the hand-held mission materials came from these software programs, but rarely was it limited to that.
It could be up to 5-6 hours of planning preparation for a single mission that might include 1-10 aircraft.
Mission planning for the U-2 was very similar, even using the same software (AFMSS), and the same concepts used in fighter/bomber attack planning except applied to the various sensors the airplane was equipped that day, and what specific products the Intel cell was looking for.