*In the summer of 2009, my friend Mia and I rode across the country on the TransAmerica Trail.
We soon figured out that we could count on 10mph (including stops for food, bathroom) to figure out how far we'd be going that day. In a typical week, we'd average 63 miles/day, including our 1 day off. So we'd ride 45, 65, 85miles/day and 300-400 miles/week. We never learned to get up super early, so we never rode more than 85 miles in a day. With 2 more morning hours, we could have broken 100. With touring, it's not about being fast on the road-- it's just the time you spend on the road.
You've also got to accept that weather and terrain will have a huge influence over your plans. We tried making plans to meet up with a friend of ours in Idaho.. and ended up arriving 24hours later than anticipated. Thunderstorms, mountains, headwinds, flat tires, chaffing thighs, brutal sun.. there are a lot of things that can slow you down!
Remember-- you're not just out for a bike ride and will be back home in a few hours.. you are LIVING on the road!
So-- don't get too cocky when planning out your mileage. You might feel comfortable hopping out of bed and whipping out 60-80 miles on your carbon fiber road bike.. but try 55 miles on your steel rig, with Marathon Plus tires and 40-50 pounds of extra gear/food/water, making an 80-90 pound boat to steer up the mountain pass instead of a 17 pound whip.
If I were to do the math for my next multi-month tour, I'd plan an average of 55 miles/day (knowing that this includes the 1 day off/week) instead of 65. I'd ride earlier in the morning and have more time to relax in the afternoon. Even if I rode farther/faster, it's good to have some 'buffer' days for unexpected delays or opportunities-- like that shredded tire or blue grass festival.
For tours a month or less, I'd be tempted to plan a higher average-- but I'd use the extra time to smell the flowers.