I think it's awesome that you are only 12 years old and already a "bikie" and a dedicated athlete! I was totally clueless when I was 12 back in 1972. 50 miles may be difficult near the end but the human body is capable of more than it thinks when it wants or has to succeed. Consider this to be a real opportunity to learn about your cycling strengths and weaknesses. Fuel and water will be important. Also, be tuned into your comfort level (clothes, position on the bike, tight muscles). Take it slow the next day because your body will benefit from a gentle workout and some down time. I admire you. So many kids are lazy and overweight these days. Enjoy the freedom that riding provides. Be proud of your fitness! I work full-time nights as a psychiatric nurse and I've ridden over 1200 miles this year.