The split long run isn’t a full-on replacement for all of your long runs—it’s an effective alternative to doing one huge long run of 2-plus hours every week, where injury risk increases as form breaks down and fueling tends to suffer. QT2 athletes, for example, rarely run longer than 2 1/2 hours in a single session, but might total three hours of running in a single day using the split long run.
Sometimes life forces you to split up long runs. This is very likely in our house. We will try to purposefully incorporate these into our training regimen more often. There are benefits and we hope to see those as our marathon training progresses.