re: Running Mileage per Week

Im 57, and I am up to 6 mi. a week. Only run 3 times, usually mon, wed, fri. I didn't run fri, golf trny., beer drinking, so I ran sat. I need to get some cardio in on the off days and have a mountain bike to do so with. My knees aren't the best, but have no real pain while running, but wanna give em a brake. How many miles would I have to ride to equal 2 miles of jogging? And I know its about the getting the heart beat up, and my runs arent at snails pace, and have several hills on my route. And I hope to up to 9 miles a week running. Just started back a month or so ago, running. Thanks In Advance gurus of health.