The highly ionized warm/hot intergalactic medium (WHIM) permeating the vast space of the cosmic filaments is one of the least explored baryon contents in the local Universe. Technological limitations hamper X-ray emission studies suited to finding the warmer phase of the WHIM. Therefore, they have been focused mostly on volume-limited astrophysical objects, such as individual <10 Mpc bridges connecting galaxy cluster pairs or in the outskirts of massive galaxy clusters. The properties of the WHIM in long cosmic filaments with lengths reaching up to 20-100Mpc, which account for >80% of the total filament volume and 50% of the baryonic mass, remain to be explored. In this work, we examine the emission properties of the WHIM in long cosmic filaments using the deepest SRG/eROSITA All-Sky Survey data available. By stacking the X-ray emission from 7817 cosmic filaments traced by the galaxy distribution by the Sloan Digital Sky Survey, we detect the X-ray emission in the filament extending out to 10 Mpc from the spine at an 8.9 sigma significance. After correcting for the emission from the unresolved point sources and extended low-mass haloes, the detection of the WHIM component remains statistically significant at a 5.3 sigma level. We further stack the eROSITA X-ray spectra and obtain the first-ever broadband 100 eV resolution spectrum of cosmic filaments at the source frame. The best-fit mean gas temperature of the WHIM component is 10^6.8 K.