I ran a marathon last year. I trained for it without have a proper base. I only ran about 9 miles a week, if that -- and my routine was rather inconsistent.
As a result, I had problems with pain and blisters and my joints through the entire training. In order to train for and run a marathon successfully, you really need to have a base of at least 15 miles a week during a consistent running program for at least 6 months.
My knees STILL hurt, and the marathon was last October. Still, I love running. I just can't get the same workout from biking or aerobics classes or anything like that.
When I went to the REI flagship store in Seattle, I happened to see an announcement for a class on trail running. The description boasted that trail running was pretty joint friendly. I was intrigued but only passing through so I didn't attend the class. Still it stayed with me.
When we got back from our vacation, I bought a pair of trail running shoes. And I've been doing it ever since. I went for a run today. :-)