The journey between Durban and EL is not that easy to split in half. Port St Johns is not actually en route (at least not on the main N2 route) and the accommodation options in Umtata are not extensive. I'd suggest leaving Durban after lunch - taking the afternoon to drive down the N2 coast road heading inland at Port Shepstone and travelling via Izingolweni and Harding to the area around Kokstad - all very scenic and doable in 2 1/2 hours. Spend the night in one of the many B&Bs in and around Kokstad. Leave early the next morning for the 6 hour drive through to EL. There is a whole accommodation industry around Kokstad built on exactly the trip you are planning.
The alternative is to continue past Port Shepstone and head to Port St Johns via Port Edward on the R61 via Bizana and Lusikisiki (I road I don't know). This is a slower road apparently. I fear the accommodation options in Port St Johns don't get great TA reviews. From there you would head inland to rejoin the N2 at Mthatha.
Either way a fascinating drive - one word of caution -- roads in the old Transkei i.e beyond Port Edward or beyond Kokstad feature lots of pedestrians and are unfenced, so there is also plenty of livestock on the road. I strongly recommend that you drive the route during daylight hours.