Welcome to the madness! They don't call them bricks for nothing- my legs feel more like massive boulders when getting off the bike. Do them once a week or more (a shorter and a longer one for variety), and your legs will grow accustomed to the feeling and muscle memory will build fast.
You'll have so much fun during your first tri. Just remember that you'll never have that first experience again, so enjoy the day to the fullest.
Glad you're here!