I have gone on group rides about 30 miles long after my commute, which is also about 30 miles round trip. I was dragging the pack on hills and pretty wasted when the ride was over. However, my biggest constraint is time. In order to join group rides after work, I have to rush home, eat something real quick, pack my bike gear in the car and drive to the group's meeting point -- which is hard to do during the limited amount of time between the end of my work day and the start of the group ride. I have also tried riding directly to the group ride during my commute home, but that makes for a very long ride carrying all of my commuting gear -- plus I would end up riding home in the dark. As a result, I rarely participate in weekday group rides any more. About the only times I do are on days when I can't bike commute for some reason, or on days when I drive because it's raining in the morning but the weather clears off by the afternoon.