Is walking 20-26 miles per day in about 7 hours for 5 or 6 days per week too much?

The answer depends on your state of fitness, the weather, your motivation and if you are carrying a heavy rucksack. 20 miles in 7 hours is 2.8mph and reasonably comfortable for a fit person. 26 miles in 7 means a much harder pace of 3.7mph and it takes a concerted effort to bash out this pace. The Royal Marines final test is a 30 mile yomp over Dartmoor in 8 hours at 3.75mph. They do it carrying 18kg or about 40lbs.

The problems arise when doing this on consecutive days. Back to back 20 milers get tiring. Up this to 26 and it gets hard. But if you train for it and get fit it is possible. John Merrill, the first person to walk the entire British coastline would often put in back to back 30 milers at a good pace with 50lbs of kit.

Personally I would only do 3 days of 20 milers if I had to. Knocking things back to 15 makes any walking experience more of a pleasure.