Talking is easy, but many things are hard to put into practice.
Literal
Speaking [nominalizer-の] [topic-は] easy but, execution [possessive-の] difficult things [topic-は] many exist.
A proverbial observation on the gap between words and deeds. が ('but') cleanly contrasts the two halves. 実行の困難な事 is a formal noun phrase where 困難な modifies 事 with 実行 specifying the domain. The balanced, essay-like structure marks this as written register.