  During solar flares, a large flux of energetic electrons propagate from the
tops of reconnecting magnetic flux tubes toward the lower atmosphere. Over the
course of the electrons' transport, a co-spatial counter-streaming return
current is induced, thereby balancing the current density. In response to the
return current electric field, a fraction of the ambient electrons will be
accelerated into the runaway regime. However, models describing the accelerated
electron beam/return-current system have generally failed to take these
suprathermal runaway electrons into account self-consistently. We develop a
model in which an accelerated electron beam drives a steady-state, sub-Dreicer
co-spatial return-current electric field, which locally balances the direct
beam current and freely accelerates a fraction of background (return-current)
electrons. The model is self-consistent, i.e., the electric field induced by
the co-evolution of the direct beam and the runaway current is considered. We
find that (1) the return current electric field can return a significant number
of suprathermal electrons to the acceleration region, where they can be further
accelerated to higher energies, runaway electrons can be a few tens of percent
of the return current flux returning to the nonthermal beam's acceleration
region, (2) the energy gain of the suprathermal electrons can be up to
$10-35$\,keV, (3) the heating rate in the corona can be reduced by an order of
magnitude in comparison to models which neglect the runaway component. The
results depend on the injected beam flux density, the temperature and density
of the background plasma.
