Girlfriend wants me to join her horseback riding even though I havent ridden one before?

My gf is a real horse enthusiast and happens to own horses herself. She spends any spare time riding her horse since she lives on farm land. It is a big passion of hers and we have a lot of conversations about her mastering jump courses and whatever skills she is working on. I even went to her horse show last weekend and watched her compete. Here is the problem, she wants me to ride a horse with her by some lake near her house on Saturday and have like a picnic. It sounds really fun and romantic. The problem is that I have never been on a horse before and I am actually a little scared. I guess I am asking, is it difficult to ride a horse at walking pace? I don’t want to let her down and when I told her I never been on a horse but want to learn, she said its super easy to (trail ride). So should I be a little nervous riding a horse for the first time or am I overreacting?