Using a Mountain Bike on the Road?
If you have a mountain bike that you mostly use on the road, consider changing to slick/road tyres from your existing knobbly mountain bike tyres. The traditional mountain bikes tyres are generally not made for road use, especially when you plan on covering some long distance spins. Changing your tyres will decrease your rolling resistance on the road and make travelling by road on a mountain bike feel like less work.
Before buying your new tyres, check your existing tyres to get your wheel size, it is normally 26 inches on a standard mountain bike. Your local bike shope will be a good place for advise on the correct tyres and bike setup you need to suit what type of cycling you do. Changing will make a big difference.