I have knee issues and one of my commutes is 17 miles one way. The way I trained for it was by building up some base miles. I stuck closer to home first. My first ever run was 8 miles. Then I started slowly building up from there. They say to increase your mileage by 10% at a time. Since you have knee issues like me I'd say go for easy-does-it. Use those low gears so you aren't pushing really hard on any hills. If you keep your legs moving at about 80 rpm you should be in good shape. Ice your knees after a long ride. That will help with the swelling. You can also work on speed while working up your base miles but since you aren't exactly racing that is flexible.