I got my first bike today since I was a kid. I don't know anything about bikes so I just bought the cheapest one I could find. Its a mountain bike. I'm fairly fit I run 5k three times a week. I went for my first cycle today and lasted about 5mins!! my butt hurts and my upper legs are in agony. I'm just wondering is it me or the bike? i thought I could do at least half hour for my first cycle. Now im not out of breath my legs just wont go! where am i going wrong.