100 miles a day is a lot. However, it is perfectly sustainable.
Keep in mind, people walk the whole Appalachian trail every year walking 20 miles a day with a lot of weight on their backs, which is more of a beat down on your body than a bike.
If you aren't doing 100 mile days often, your legs will get really, really sore on about day 3, like too sore to walk. The more often you put in that type of mileage the less your legs will hurt a few days after starting.
Thing is, your body is insanely adaptable. If you have the mental strength to push through the pain, your muscles will come around and the last 3 days will be a really good time.
The first time I did a century a day for 7 days I was in no shape to do it. The first 3 days sucked. The last 3 were a great time. That middle day was a wierd one. Do not underestimate your body's ability to get in shape on the trip.
But train anyway!!!!