I come here as a complete know-nothing as far as cycling goes; long story short, I've decided to get fit and lose some weight and interested in cycling for exercise. I currently have an older (2004-5ish) Gary Fisher mountain bike and am wondering if 1) I can put road tires on that and be good to go or if that's not possible, 2) suggestions for a new road/hybrid/fitness bike. Not looking to do any serious long-distance cycling or racing, just something I can do some riding on (lets say no more than 10 miles a trip) to get into shape.
Any info is greatly appreciated and I'll pre-apologize if I used incorrect terms or don't know them.
TL;DR: For getting in shape, road tires on a MTB I already own or suggest a new road/hybrid/fitness bike, budget $550-700.
EDIT: Tires/Tubes/Tools purchased, waiting for day off to clean path in the garage to get to it and swap tires and get riding! Thanks for all of the info and help!!