Wavy dynamics of thin films with moderate Reynolds number shows a remarkable feature: there appears to be a natural wavelength between solitary-like pulses far downstream. This wavelength is much larger than the average film thickness, and, for long systems,does not depend on system size. The phenomenon is robust, resembles the inverse energy cascade in turbulence, and probably is the most interesting in the interfacial dynamics of thin films. A new simple approach is proposed for the description of the interfacial dynamics in film flows, which captures such an inverse cascade. The essence of the method is in combination of the pertinent dispersion relation, evaluated exactly by numerics, with the simplest quadratic nonlinearity. It turns out that the exact dispersion relation differs dramatically from the conventional asymptotic dispersion relations (e.g., in KS equation, or boundary-layer theories). This is the reason why the conventional long-wavelength evolution equations were not successful in description of the wavy film dynamics for moderate Reynolds numbers.