One of the most daunting things about running an ultramarathon is combing through the blogs and magazines and finding that everyone seems to be running 100 miles per week. First, let me dispell that myth. Not everyone who runs ultramarathons is putting in 100 miles per week. I ran my first 24 hour race, although I only finished 12 hrs, on 25 miles a week. I did supplement with biking miles, but it can be done. I should note that I have never been more sore during and after a race. I myself max out somewhere around 60 miles per week. That still sounds like a lot, but you must realize that’s 1-2 weeks out of the training cycle. A friend of mine has won several races on 40ish miles per week. I say this not to discourage you from upping the mileage, but to point out that it can be done on less. More mileage tends to result in more fitness which can help you run a race faster.
Back to the main point, building mileage. How did I go, or how does anyone go from running 20ish miles a week to running 60+ without getting sick or injured? The coaching manuals out there tell us to limit increases to 10% per week to avoid injury. Based on my experience, it doesn’t matter what you limit your increases to if you have some sort of underlying problem. So the first thing I would recommend is finding someone to analyze your running gait, and check your arches. There are numerous athletic trainers out there who can do this. Particularly I would make sure they look at lateral knee motion, and arch movement. They can give you exercises to strengthen the stabilizing muscles which the majority of the time is the source of the injury. If you don’t want to go that route, find a good core strengthening routine that hits all the glutes and tiny hip muscles. This might be one possible reference
For arch problems, I would recommend find a good podiatrist. I discovered that my arch would collapse whenever I put weight on it. As a home remedy I used a rigid arch support. The orange insoles are my favorite because they have a little cushion. The greens are like running on concrete.
Another option is to look at shifting your stride away from an aggressive heel strike. Whenever the heel strikes in front of the body it places excessive force on the load bearing joints, muscles, and bone which can result in injury. This may require you to hire a coach, and frankly will require more writing in the future.
So the real secret to increasing your mileage is to minimize your risk of injury by first training the musculature, and if needed, to correct structural problems with orthotics.