We present the first experimental demonstration of a resolvable mode structure with spacing $c∕2nL$ in the RF spectra of ultralong Raman fiber lasers. The longest ever demonstrated laser cavity $(L=84\mathrm{km})$ , RF peaks of $\sim 100\mathrm{Hz}$ width and spacing $\sim 1\mathrm{kHz}$ have been observed at low intracavity powers. The width of the peaks increases linearly with growing intracavity power and is almost independent of fiber length.