We develop a dynamic multi-band OFDM subcarrier allocation scheme to fully utilize the available bandwidth under the restriction of dispersion- and chirp-related power fading. The experimental results successfully demonstrate an intensity-modulation-direct-detection 34.78-Gbps OFDM signal transmissions over 100-km long-reach (LR) passive-optical networks (PONs) based on a cost-effective 10-GHz EAM and a 10-GHz PIN. Considering 0-100-km transmission bandwidth of a 10-GHz EAM, the narrowest bandwidth is theoretically evaluated to occur at ~40 km, instead of 100 km. Consequently, the performances of 20-100-km PONs are experimentally investigated, and at least 33-Gbps capacity is achieved to support LR-PONs of all possible 20-100-km radii.