100miles a day is do-able if you are in pretty decent shape. I averaged 140-150kms (80-90miles) a day on my cross Canada tour in 2005, although I started slower due to crossing the mountains and rains. I think the keys to success are:
1)1250 miles, would be 12-14 days, a longer tour might really wear you down, for a 12 day I donít think a rest day would be needed, I started the tour off with days of riding without a day off, 1300 miles, I did a couple of days light days of 55-65miles, and two tail wind days of 140-160 miles. On average it was 8o miles a day. Towards the end of the journey the average was up to 100 miles a day.
2) Start early, end late. In my first 2 weeks I had a couple of late starts, and early finishes, on average I was on the road for 10 hours a day for this section, by that I mean I started the clock when I started riding, and stopped when I started setting up camp, so that 10 hours includes all breaks as well. Starting and hour earlier and ending an hour later would give you all the time you needed
3) Avoid restaurants, a lot of my 'on the road' time was spent in restaurants where a meal could last an hour by the time you sit, order, wait, eat, wait, pay, leave. Eating while riding is the fastest way, but never leaves me full.
4) Pack light, no brainier, leave the gadgets at home, if you are riding long days you wonít have time for them anyways.
5) Try aero bars, I have yet to try them, but I rode with a fellow that had them on his 29incher MTB bike. When he rode normally we went at the same pace, the second he dropped into the aero bars he gained 1-3 mph and easily pulled away from me. If your set up allows them, and you can find a comfortable position, then I would recommend them. 1-3mph over 10 hours is 10-30 miles.
6) Get into a good routine with setting up and breaking down the camp, longer days on the road means you have less time to set up, eat, sleep and break down. If you are cooking, break out the stove and start cooking as you set up your tent. Do the reverse in the morning when making breakfast.
7) Diet would be key, my diet is absolute crap.. so I am not the best person to give advice, but I would bring a hydration drink like eload. I'd also eat a lot of protein and salty foods at the end of the day to help with muscle recovery, hell Iíd just eat a lot of everything. I'd also take some vitamins to prevent muscle cramps.
8) Donít be afraid of the dark, just be prepared for it. Not only is it cooler at night, there is normally less traffic. I used to be worried about riding at night, but with a good mirror you can see a car coming from a mile away, and watch to see if they pull over to avoid you. Iíve never had any close calls at night when well lit up. Make sure you have a back up lighting system as well. Services are often closed at night, make sure you have more than enough food and water.
I think desire is the biggest thing that will help you, if you desire to push yourself, to stay on the road longer, to cumulate those miles, than you are already a step ahead on your goal.
Every year I do a 2 day, 525km (325mile) tour partly loaded. I mostly do it to push myself and to see what I can do at my limit. This year I crawled on the second day in 90 degree heat, and a good head wind. But in the end it was the type of challenge that I look forward too once a year. 2003 trip can be found here: http://www.crazyguyonabike.com/journal/Kingston2005