Hmmm.... there are 2 tire sizes we're talking about (and please forgive me if I'm telling you things you know) - the wheel circumference and the width of the tire. Most bikes are either 700c or 26" in diameter. 700c wheels are the larger wheels you usually see on road bikes, and 26" wheels are probably what's on your mtn bike (unless you have a 29'er).

Most road bikes come with 23c width tires. Very narrow. Most won't fit more than a 25c or a 28c. Probably only 23c if you want to run fenders. The cross check has the larger, 700c wheels, but will run wider tires. I have run 35c and 28c tires on mine. The 35c is plenty wide for me on gravel roads. Until you get up to large frame sizes, the LHT will have 26" wheels. I have 32c tires on my Trek 520 (they're stock). Your LBS is probably recommending a cross bike because it will take wider tires. Touring bikes would work too. They aren't a bad choice for road riding - just wanted to make sure there was a reason

Some folks care a lot about weight of the bike, some not at all. I think it, in part, depends on your weight/height. 5 lbs of bike weight will be less significant for someone who weighs 250 pounds than someone who is 115 lbs. And there's a bit of philosophical debate over it.

Short term - try putting slick tires on your mountain bike. You can still get them fairly wide, but you'll pick up a lot of speed on the road, and your training will likely get easier.

Good luck!

CA