The interstellar medium (ISM) has been treated by interstellar mission planners as an obstacle to be survived rather than as terrain to be read and navigated strategically. We propose a reframe: the ISM is navigable terrain with measurable variation in density, ionization state, magnetic field structure, and plasma conductivity that has direct consequences for both propulsion efficiency and radiation exposure along any interstellar trajectory. Drawing on three-dimensional Gaia-derived ISM dust and ionization maps, heliospheric boundary data from Voyager 1 and 2 and the IBEX mission, and current propulsion concepts including the Solar Oberth maneuver, magnetic sail, and laser lightsail, we argue that trajectory optimization for minimum ISM resistance and maximum plasma conductivity is a tractable near-term research problem that has not been formally addressed. We identify the Beta CMa tunnel as a candidate low-resistance departure corridor, evaluate three propulsion architectures against ISM terrain criteria, identify the room-temperature superconductor problem as the primary engineering bottleneck for magnetic propulsion in the inner solar system, and propose a dedicated Interstellar Terrain Mapping mission as the necessary Phase 1 precursor to any optimized interstellar departure. This paper is the second in The Traveling System series. The companion paper establishing the galactic forcing framework is available at https://doi.org/10.5281/zenodo.19717287