I can assure you that if you want to do mileage on the road then a road bike is the best bet. I hav done Century rides on MTB's as that was the only type of bike I had and the only problem I had on a long ride was speed. Comfort was there and the gearing for the hills.

I rode MTB's for 16 years before changing over to road bikes. What a difference. I found different muscles- My back ached and the neck and shoulders were agony--for a couple of months. I know that people will keep telling you that Road bars have different hand positions so you can keep changing posture but so does an MTB with bar ends.

When I went into the LBS to buy that first road bike- They told me to buy cheap but don't buy cr*p. Bought a Giant OCR3- A good bike with lowly components- As all that first bike was there for was to tell me what the 2nd bike was going to be. They were right and I used that OCR to learn road riding- find the faults with me that would have to be corrected and to find out what I wanted to change or modify to improve my road riding skills. They were right as a year later N+1 Came along in a bike that fitted perfect- had the right bits fitted to it and was really comfortable.

The lecture is over with but I can assure you that if you are changing style of bike- then the LBS can help you greatly. Depends on your budget but remember that Road bikes come in several grades of quality- ridability- fit and comfort. Don't get set on one model or one manufacturer though. Get a few test rides and compare what is in the market.