Hi,
Weight loss is greatly dependent on your diet.
As long as your total calorie intake is lower than your total calorie output, you will lose weight.
You can't out-exercise a bad diet. Recent studies have shown that most individuals actually burn much less calories than what the HR monitors or exercise machines display.
Also, 20miles a day is alot of distance to cover, especially if you're just starting out. This will make it hard for you to keep up consistently. I would advise you to cut calories and embark on a easier workout routine to kick start your weight loss