A peninsula is a piece of land that is surrounded by water on three sides. It's connected to a larger landmass (a continent or a larger peninsula). There isn't one "main land" of a peninsula - the peninsula itself is the piece of land that is connected to the larger landmass.

For example, Florida is a peninsula. It's surrounded by water on three sides: the Atlantic Ocean, the Gulf of Mexico, and the Florida Straits. It's connected to the mainland United States.

If you can tell me which peninsula you're interested in, I might be able to give you a more specific answer!