Having read a little more:

Ryan's original food plan was as follows:
Week 1: ~ 1.5 lb of food / 3,000 Calories
Week 2: ~ 2.0 lb of food / 4,500 Calories
Week 3: ~ 3.0 lb of food / 7,000 Calories
Last 3 Days: ~ 3.2 lb of food / 7,300 Calories
+ 2 pounds of peanut butter as a reserve.
+ 8 pounds body fat and 4 pounds body protien in 21 pound weight loss
155 pound initial body weight at 17% body fat.

Week 1: 20 miles/day + 1200 ft/day cumulative elevation gain
Week 2: 30 miles/day + 2700 ft/day cumulative elevation gain
Week 3: 45 miles/day + 4900 ft/day cumulative elevation gain

So by adding more week 1s then perhaps it could be extended:

Scaling up to 180 pounds person with 20% body fat, starting out.
24 days = 600 miles, 10 pounds gear, 25 pound weigh loss, 60 pounds food.
40 days = 800 miles, 10 pounds gear, 30 pound weigh loss, 90 pounds food.

Actual mileage would depends alot on terrain of course, and climate, and fitness. I would be just as happy to do the 600 miles in 40 days. I'll bet those voyageur guys from the past could do 1000 miles in 40 days with 100 pounds though. I understand the best of their breed were actually somewhat on the smaller side. I'm not sure what role age might play into it. It would probably mostly effect the weight you could start out with, but I'll bet if you weren't in a hurry you could still do 400 miles in 40 days without resupply at a fairly ripe old age.