Answer:
The sunny weather allowed him to play outside all day.
Step-by-step explanation:
Dylan Thomas' poem "Fern Hill" is a reminiscing of the poet's childhood days at his aunt's farm at Fernhill. The poem gives a detailed and idyllic description of how the place looked and how he used to enjoy himself.
The lines given in the question are from the third stanza of the poem where he described in an expanded detail, the idyllic, Edenlike setting of the farm. He used allegory and even metaphors to compare the from to that of the Garden of Eden. When he said "All the sun long it was running", he meant that thew god, sunny weather of the place allowed him to be outdoors all day. He could play all day and be around the nature, without any disturbance.