I think you will find these statistics of Lion King's hike of both the northern and southern routes of the American Discovery Trail illuminating. http://www.trailjournals.com/stats.cfm?trailname=5876

He hiked 6,170 miles in 425 days, averaging 16.54 miles per hiking day and 14.52 miles per day including rest days.

I like trekking poles for walking on trails as they help prevent falls and make walking downhill on a slippery trail easier. They also absorb the impact while walking downhill. However, my suggestion for use on roads was not based on opinion. It was based on credible scientific studies that showed that the the most efficient way to walk on roads is to keep the upper body as erect and as motionless as possible. If a person was so inclined, the best way to learn the technique is to carry something on their head. This position increases the leg's efficiency from about 65% to about 85%. By efficiency, I mean the fewest calories per mile which is measured by oxygen use.

Since there is plenty of time to walk 20 miles in a day, there is no reason to increase speed too much. After practicing for a few hundred miles, a walker will probably average about 3.7 mph plus or minus a couple tenths. Efficiency starts to decline while walking above or below 3.5 mph, and this seems to be independent of a person's height for reasonable variations around 5'10 inches. I know this is counterintuitive, but that's what the studies show.

It may seem like over thinking to worry about efficiency, but if a person does not completely recover after each day before starting the next day, the effects will be cumulative. The least serious result will be forcing a rest day. The most serious result is an injury which could require surgery in time. In one of my first posts on this thread, I recommended starting to seriously train as soon as possible. It will take a year to achieve all the needed physiological changes in your body. By serious, I don't mean starting with high mileage. I mean starting with what you are capable of now, and gradually increasing your mileage.

Diet is another issue. I don't want to get in the middle of a diet discussion. I suggest reading "Eat and Run" by Scott Jurek who won the Western States 100 five times.

I'm not optimistic about the idea of averaging 20 miles a day. Although people do it on the Appalachian Trail, I haven't found many modern road walkers that can do it. There are modern runners that can average this distance, but most of these have had various operations as a result of injuries.