  We obtain the exact asymptotic result for the disorder-averaged probability
distribution function for a random walk in a biased Sinai model and show that
it is characterized by a creeping behavior of the displacement moments with
time, <x^n> ~ t^{\mu n} where \mu is dimensionless mean drift. We employ a
method originated in quantum diffusion which is based on the exact mapping of
the problem to an imaginary-time Schr\"{odinger} equation. For nonzero drift
such an equation has an isolated lowest eigenvalue separated by a gap from
quasi-continuous excited states, and the eigenstate corresponding to the former
governs the long-time asymptotic behavior.
