Whereas Holmes and Moldvay Basic concentrated on dungeon/Underworld adventures, Cook/Marsh Expert focused on Wilderness adventures, the counterpart to the Underworld in OD&D vol. 3 "The Underworld & Wilderness Adventures".
Much of "Part 4: The Adventure" is drawn from OD&D vol. 3. It's interesting to compare the two, to determine what was included, and what was left out.
Cover illustration for the Hexagonal Mapping Booklet (1980/1), by Bill Willingham
Base towns are defined as "a place where the party can buy equipment and hire men and retainers, where treasure can be sold (and perhaps magic items identified), and where clerical healing is available for a suitable "contribution" or service."
Organizing a Party:
Steps for journeying into the wilderness are given:
1. Deciding where to go.
2. Deciding what supplies are needed.
3. Equipping and outfitting the party.
4. Establishing a marching order.
"Hexagonal-grid maps are the best because there are six spaces to go to that are an equal distance from the space started from; with a square grid there are only four (diagonals distort the distance)."
Time, Scale and Movement:
Conversion of movement rates in the dungeon (in feet) to movement rates in the wilderness (in miles) is accomplished by dividing the number by five. Pursuit speed is also covered.
Penalties for moving through different types of terrain (ultimately derived from those in "Outdoor Survival"), as well as rules for forced marches are given.*
*OD&D vol. 3 also specifies a 1 hex penalty/day for parties numbering over 100, including pack or draft animals, and a 2 hex penalty/day for parties numbering over 1000
OD&D vol. 3 provides information on the type of guards/retainers by character type, together with additional information regarding each character type:
Fighting Men: will demand a jousting match, or else a 100-600 gp toll, (rules for jousting from "Chainmail" are recommended; see also "Chainmail Revisited: Jousting in D&D" by Jon Pickens (The Dragon #17, Aug 1978)
Magic-Users: will send passerby after treasure, using "geas" (with magic-user claiming at least half of all treasure so gained, as well as the first choice of magical items), or else demand a magic-item or 1000-4000 gp as toll
Clerics: will require passerby to give a tithe (10%) of all money and jewels, or send on a Lawful or Chaotic task, using "quest" (evil high priests may simply attempt to slay any Lawful or Neutral passerby, who don't pay a tithe)
Covered in "Part 9: Special Adventures".
Traveling by Air:
Guidelines for the size of creatures that can be carried by an aerial monster are given.
Obstacles to Movement:
Rules for rest are also covered.
Likewise based on chances and mechanics from "Outdoor Survival" (additional information given in "Part 8: Dungeon Master Information" (pg. X56)
Specialists and Mercenaries:
Specialists mentioned in OD&D vol. 3, not included in the Expert rulebook:
Assassin (2000 gp/mission): The role of this hireling is self-evident. The referee will decide what chance there is of his mission being accomplished by noting the precautions taken by the intended victim. Assassins are not plentiful, and some limit on the number employable during any game year must be enforced.
Smith (25 gp/month): As already mentioned, a Smith is able to assist an Armorer. For every 50 horses or mules in a player character's force there must be one Smith to maintain them.