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.
Now, no LT1 is going to be making over 2600 foot pounds of torque (except possibly for a single, glorious instant, running on nitromethane), but, assuming we needed 12 rpm for an input to the mill, we could run the LT1 at 5000 rpm (where it's making 315 foot pounds of torque), and gear it down to a 12 rpm output.