Lose 10 pounds is easy. Very easy. Can be done in about 5-6 weeks. The hard part is keeping it off because once you did it then that goal is met and you stop trying.
Exercise twice a week is not ideal, you’re better off doing 3 times a week (burn / rest / burn / rest / burn / rest).
Marathons can damage the body so be in a club with 'ole timers who can watch your style, so you don’t over-do it, but if you build up gently then its not that hard, mostly technique.