To lose weight and keep it off: You will need a high amount of physical activity unless you also adjust your diet and reduce the amount of calories you're eating and drinking. Getting to and staying at a healthy weight requires both regular physical activity and a healthy eating plan. The CDC has some great tools and information about nutrition, physical activity and weight loss. For more information, visit Healthy Weight. Reduce Your Risk of Cardiovascular Disease
If 10 revolutions per minute equal 1 mile per hour, then 1 rpm = 0.1mph. At 25rpm you should br producing 2.5mph, and at 35rpm you should get 3.5mph. I would suggest you measure the length or distance you are actually achieving, and see if this stacks up.
In the United States, the fatal crash rate per mile driven for 16-19 year-olds is nearly 3 times the rate for drivers ages 20 and over. Risk is highest at ages 16-17. In fact, the fatal crash rate per mile driven is nearly twice as high for 16-17 year-olds as it is for 18-19 year-olds.