  [ABRIDGED] The distance to the relativistic jet source SS433 and the related
supernova remnant W50 is re-examined using new observations of HI in absorption
from the VLA, HI in emission from the GBT, and 12CO emission from the FCRAO.
The new measurements show HI in absorption against SS433 to a velocity of 75
km/s but not to the velocity of the tangent point, which bounds the kinematic
distance at 5.5 < d_k < 6.5 kpc. This is entirely consistent with a 5.5 +/- 0.2
kpc distance determined from light travel-time arguments (Blundell & Bowler
2004). The HI emission map shows evidence of interaction of the lobes of W50
with the interstellar medium near the adopted systemic velocity of V_LSR = 75
km/s. The western lobe sits in a cavity in the HI emission near the Galactic
plane, while the eastern lobe terminates at an expanding HI shell. The
expanding shell has a radius of 40 pc, contains 8 +/- 3 x 10^3 M_sun of HI and
has a measured kinetic energy of 3 +/- 1.5 x 10^{49} ergs. There may also be a
static HI ring or shell around the main part of W50 itself at an LSR velocity
of 75 km/s, with a radius of 70 pc and a mass in HI of 3.5 - 10 x 10^4 M_sun.
We do not find convincing evidence for the interaction of the system with any
molecular cloud or with HI at other velocities. The HI emission data suggest
that SS433 lies in an interstellar environment substantially denser than
average for its distance from the Galactic plane. This Population I system, now
about 200 pc below the Galactic plane, most likely originated as a runaway
O-star binary ejected from a young cluster in the plane. New astrometric data
on SS433 show that the system now has a peculiar velocity of a few tens of km/s
in the direction of the Galactic plane. From this peculiar velocity and the
symmetry of the W50 remnant we derive a time since the SN of < 10^5 yr.
