Fighting is the best way to level up. Did you find the Ancient Maze? It is worth 3 to 4 XP. Add to xp is runnng all of arcanum in the World Map but set up like a spreadsheet. You will find many creatures to fight and learn all sites in Arcanum. If your willing use a mod its level 127 here at Terra Arcanyum, click on Factory then click on hacks and you see the level 127 and down load read the readme and it will tell you where to put it. If you have Windows XP then you need to install it but the change if folders in XP will need some custom installation because the readme only talks about windows 98.
try these areas do the closed sewers and the wolf cave, and the ancient maze if you want combat experience quickly, just make sure you have virgil as your healer
Also make sure you find someone competant to be your healer when you level high enough to have a productive follower.