Your legs are still fresh and your head is full of the excitement of setting off. That’s exactly the danger of a first day: you end up walking faster than is good for you.
From the hill of Vézelay, where the basilica waves you off, you descend into the countryside. For the rest of the day the path rises and falls through woodland and pasture and past small villages. No single climb is nasty, but the 490 metres or so of ascent really add up. At over 7 hours of walking, this is a hefty introduction. Anthien is a small village, and you arrive with heavy legs and a contented feeling. The journey has begun.
Take it easy, the road to Saint-Jean is long.