  We study the three-dimensional persistent random walk with drift. Then we
develop a thermodynamic model that is based on this random walk without
assuming the Boltzmann-Gibbs form for the equilibrium distribution. The
simplicity of the model allows us to perform all calculations in closed form.
We show that, despite its simplicity, the model can be used to describe
different polymer stretching experiments. We study the reversible
overstretching transition of DNA and the static force-extension relation of the
protein titin.
