This is normal. Whether your clothes fit for the first time, then whether your seat fits, all the equipment is in the running-in stage, and your body function is also in an adaptation stage. It's like you don't exercise for a long time, you don't run for a long time, and then you suddenly exercise and run. You will find that the muscles are particularly painful the next day, sometimes even lasting for a week. So I suggest you step by step, or ride a short distance first, and then slowly increase the riding distance. Let your body and all your equipment have a gradual adaptation process. This will be much better.