Small but important correction: the actual quote is “Soylent Green _is_ people!”. Agree with your broader point about the Torment Nexus being a McGuffin, but we do actually know how Soylent Green is people in the film - it’s explicitly revealed that it’s made from processed corpses due to overpopulation.
Worth adding that there are a bunch of paid-for apps offering 'couch to 5k', but the 2012 podcast version by the NHS was such an elegant and effective idea—essentially, a free audio coaching program of progressive run-walk training.