Standard multimode optical fibers normally support transmission over some 100 modes. Large differences in the propagation constant and the spatial distribution of distinct modes degrade the performance of phase-sensitive optical time-domain reflectometry measurements. In this work, we present a new realization of a coherent time-domain interrogation technique using single-mode operation in multimode fibers. We demonstrate effectively distributed strain sensing on three different multimode optical fibers. Up to 4 km of multimode fiber has been correctly interrogated, featuring a spatial resolution of 20 cm.