Improving the flexibility of active distribution networks (ADNs) enhances the reliability and economic performance of regional power systems. Common user-side flexible energy resources (FERs), which are numerous but individually limited in capacity, can be aggregated to participate in ADN operations. To accurately and efficiently aggregate heterogeneous user-side FERs, this paper proposes a novel polytope-based aggregation method to construct the aggregated feasible region, which is integrated into a coordinated operation model for ADNs. The proposed method employs an inner approximation using affine transformation to better capture the feasible regions of individual FERs. In addition, a distributionally robust chance-constrained (DRCC) model is integrated to address parameter uncertainties in FERs, and a rolling mechanism is applied to intraday operations for the rolling update of the aggregated feasible region. Finally, a case study of a specific ADN in Northwest China validates the effectiveness of the proposed method and the corresponding improvement in flexibility. The results indicate that the proposed coordinated operation model effectively improves the flexibility of ADNs, and the proposed method reduces the computational burden without compromising economic benefits.