Oh! Yes! Saddle sore! How common is that!! Well, I can say that once a week would give you the highest instance of saddle soreness, as the riding would not be consistant enough to use your riding muscles enough to condition them properly. "weekend warrior" symptoms can be painful, indeed (at any age!). That said, before riding, or doing any exercise, it is important to warm up your body with some light stretching exercises, and repeat after exercise as a cooldown.