The peninsula in Europe that had Greek colonies that were the farthest away from mainland Greece was the Iberian peninsula. In fact Iberia is an adaptation from the Ancient Greek "Ιβηρα".
The Greeks reached the peninsula after having heard from the Phoenicians. While the greek colonial enterprise in the Iberian peninsula was more modest than the phoenician, they managed to establish several colonies. The main one was Emporiae, modern Ampurias in 575 BC and other minor colonies were Alonís and Hermeroscopeion.
The Iberian peninsula and mainland Greece are separated by roughly 3487km between Madrid and Athens.