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.
Yes, and with a 72 gear one would do more pedal strokes for the same average speed. Actually an easier way to estimate number of pedal strokes in general is to count your normal cadence (say 70) and apply it to your time to do an average mile (e.g., 4 min. for 15mph) = 280 strokes per mile. That would yield 28,000,000 for 100,000 miles.