  The most striking feature of quark distribution functions transformed to the
longitudinal distance representation is the recognizable separation of small
and large longitudinal distances. While the former are responsible for the
average properties of parton distributions, the latter can be shown to
determine specifically their small-$x$ behavior. In this paper we demonstrate
how the distribution at intermediate longitudinal distances can be approximated
by taking into account constraints which follow from the general properties of
parton densities, such as their support and behavior at $x \to 1$. We show that
the combined description of small, intermediate, and large longitudinal
distances allows a good approximation of both shape and magnitude of parton
distribution functions. As an application we have calculated low-virtuality C
even and odd (valence) u and d quark parton densities of the nucleon and the
C-even transversity distribution $h_1(x)$, combining recent QCD sum rules and
lattice QCD results with phenomenological information about their small-$x$
behavior.
