Kimo,
Howzit ? (With a name like Kimo, I'm thinking you're from Hawaii !)
My experiences are similar to jimblairo's up to (but not including) the 40, 50 & 60 miles a day.
I used to road race & tri 20 yrs ago but didn't sell my road bike. Tried to ride it after a decade and found I couldn't push the 12-23 freewheel. Got a Costco mtn bike - 27 gears and rode intermittently. Got more serious a couple of years ago and put on 90psi, 1.5" slicks on the 26" rims and started riding more. Last fall, I had enough conditioning and dropped weight from 228 to 215 that I invested in a triple, clipless pedals, and light wheels with 9spd cassette for the road bike. This enabled me to train more - even tag along with local bike club. Now I'm looking for a good road bike.
For starters, you can't go wrong with a mtn bike; just invest in some road tires and an adjustable stem for more speed, comfort, and quiet. They are heavy, mine is about 28 lbs with a chro-mo fork but I look at it as an exercise machine. Even inexpensive mtn bikes come with SIS or other indexed shifters and 27 gears. In fact I upgraded the old road bike from down tube shifters to Deore LX brifters on flat bars. Much cheaper than road brifters.
At the point where I was able to pass some road bikers, I moved to the road bike.