The implementation and assessment of sustainable initiatives at sport events has a great importance for researchers and practitioners to improve the effects on the hosting territory. All three pillars of sustainability (environmental, economic and social) must be considered. In running events, such as marathons, one of the main issues is the environmental impact of the great quantity of waste, and particularly of plastic waste. A recent tendency is to shift to a “plastic-free” model. However, some single-use plastic products have unrivalled properties and their replacement with other materials may not be the most sustainable solution, at least in the short term. In this paper, a new model for managing plastic waste at a marathon is proposed. It does not aim to eliminate plastics at sport events, but to better manage waste by increasing collection, and sorting and recycling efficiency, following the circular economy paradigm. To demonstrate the sustainability of the model, a precise and quantitative methodology is applied to an innovative visualisation tool, developed by the University of Bologna, to assess circular initiatives and some key performance indicators (KPIs) to compare the sustainability of the new model to waste management in the previous editions of the same event. With this approach, it is demonstrated that a marathon can be completely sustainable: (i) from an environmental point of view, plastic collection efficiency increased by 120.5%; the recycling rate by 157.0% and the landfill rate decreased by 75.4%; (ii) from an economic point of view, it was demonstrated that, with the technological level of the involved recycling plant, the initiative is cost-effective when the virgin PET price is greater than €776/tonne; and (iii) with regard to the social impact, it was proved that the direct engagement of participants (runners and walkers) and their positive perceptions about the initiative achieved the highest score for the two selected qualitative KPIs (3/3). Moreover, some insights have been derived to improve plastic management, covering different disciplines: technical standardisation of plastics, consumer training and legislative support for technological innovation in the industrial context.