In this paper, we propose a new relaying protocol for large multihop networks combining the concepts of cooperative diversity and opportunistic relaying. The cooperative relaying protocol is based on two diversity mechanisms, incremental redundancy combining and repetition combining. We assume that nodes in the large multihop network are modeled by a homogeneous Poisson Point Process and operate under Rayleigh fading and constant power transmission per node. The performance of the proposed relaying protocol is evaluated through the progress rate density (PRD) of the multihop network and compared to the conventional multihop relaying with no cooperation. We develop an analytic approximation to the PRD based on the concept of decoding cells. The protocol parameters are optimized to maximize the PRD of network. We show that the cooperative relaying protocol provides significant throughput improvements over conventional relaying with no cooperation in a large multihop network. It is also shown that incremental redundancy combining provides a higher gain in PRD relative to repetition combining. The gain in PRD has near constant value at all values of the path loss exponent and is monotonic in diversity order.