What is the maximum routing distance between MAC to PHY recommended?

MII and RMII have round-trip time restrictions, since the TX data path on MII is destination synchronous, and the RX data path on RMII is destination synchronous. Both RMII and MII hit a hard timing limit at a few hundred millimeters trace length.

RGMII on the other hand is fully source synchronous, which means it has no hard timing limit on the trace lengths. It simply becomes an exercise in signal integrity and length matching. With for example length and impedance matched coaxial cables and terminations you should be able to go quite far.

That said, SGMII or 1000BASE-X is better suited than RGMII for board-to-board ethernet backplane situations like this.

Without reading the datasheet, 1Gigabit is 1 nanosecond. On FR-4, with Er of about 4, the 1_foot in air becomes about 1/2_foot in epoxy-fiberglass combined with air above. You may need to tolerate round trip reflections and/or ISI energy stored in the IC leadframe and bondwires and PCB vias and Silicon ESD non-linear capacitances. Thus round trip will be 1/4 foot per side, or 3" (7.5 cm). If the link has adaptive equalization, life should be very good.