Generation I lacks later games' methods of accelerating the grinding process. There are no shortcuts here: you will need to find the highest level Pokemon you can safely KO, and farm them for several hours. Before the Elite 4, the highest levels in Kanto are found in Victory Road. I recommend going to the third floor, which has a 10% chance of encountering Venomoth, 20% of Onix, and 5% of Graveler — though restoring PP is more convenient if you stay on the first floor.
The Pokémon Mansion is a more consistent alternative to Victory Road. Though its Pokémon hover in the 30s compared to Victory Road's 40s, the latter has a higher chance of Level 20-something Zubat. Over time, you may find the uniformity of the mansion easier to work with.
Be sure to first clear the high-level Trainers in Victory Road to eke out a little more EXP. After that, settle in for the long haul.