sfrunnerdude, certain numbers do seem to be big obstacles. Whether they just work out to be where our current physiological limits are or they are some psychological limit (105 miles works out to a nice even 15 miles a day which can seem a daunting barrier until you go through it), they do seem to be real. Once you go through them, though, you can make a big jump. Keep at it. Your consistency will take you a long way even if it takes you a while to get past that 105 mile barrier.
Oh, and welcome to the site.