The cross section for pair production of top quarks (tt) with high transverse momenta is measured in pp collisions, collected with the CMS detector at the LHC with s=8 TeV in data corresponding to an integrated luminosity of 19.7 fb-1. The measurement is performed using lepton+jets events, where one top quark decays semileptonically, while the second top quark decays to a hadronic final state. The hadronic decay is reconstructed as a single, large-radius jet, and identified as a top quark candidate using jet substructure techniques. The integrated cross section and the differential cross sections as a function of top quark pT and rapidity are measured at particle level within a fiducial region related to the detector-level requirements and at parton level. The particle-level integrated cross section is found to be σtt=0.499±0.035(stat+syst)±0.095(theo)±0.013(lumi) pb for top quark pT>400 GeV. The parton-level measurement is σtt=1.44±0.10(stat+syst)±0.29(theo)±0.04(lumi) pb. The integrated and differential cross section results are compared to predictions from several event generators.