roccobike

OP, the shop trying to sell you a mountain bike should be avoided. Their statement about the "200 lb limit" is either totally mis-informed or the sales person is a liar, either is not to be trusted.
As for how long a ride you can take, when just kicking the tires and trying out a bike, a short ride is typical. When you choose one, and you're serious about buying it, tell the shop you're close to a decision, but you want to ride it a few miles or half an hour at least. I've actually had two shops want to know why I returned a bike so soon after riding for only 20 minutes. Some shops will not let you do this, but at least try.
Buying a first bike depends on your finances. If you've got the money, here's something to think about. Some folks start out on inexpensive all alloy road bikes with alloy forks. IMHO they ride like a NYC taxi with blown shock absorbers and with Claris 8 speed you get what you paid for, a low end system. If you can spring for a better bike with carbon fork, Tiagra drivetrain and maybe a little better wheelset, you're more likely to get a better idea what a quality bike is like and more likely to stay with the sport. I'd recommend you buy as high a quality level as you can afford.