>>2040113if you're going to ride a lot on roads, you can get slick tires for your mtb wheels. you will immediately notice going faster for less effort.
noobs almost always have the saddle height wrong. you want almost full leg extension with a slight knee bend so you don't destroy your knees when pedal is all the way down. you will not be able to put your feet down when stopped unless you lean over onto one foot or dismount. incorrect height can cause pain, it also saps your energy and power. it would be much more tiring to walk with your knees constantly bent, right? that's why nobody does it.
for hills you probably want to be in the middle ring up front, but you can use the little one on killer hills. on the wheel, it's the opposite: the big ones are easy and the small ones are hard but fast.
get rid of that chain. if you do it yourself you need to size it to your drivetrain with a chain tool even if it has a quick link. look up how to size it or ask us.
ass hurting is normal at first. the tissue over your ass bones needs to break in. your weight is transferred through the hip bone ends not the ass so if your saddle is the wrong size for your bone support, that can be a problem, too. there are methods to measure these things. but even with the correct size, a new rider's ass will be tender for a while.
that bike is pretty low quality but if the brakes and shifting are ok, change the chain and ride. just don't sink any money into repairs. in most used markets, you can get something decent/good for the cost of major repairs. (assuming the used bike doesn't need major repairs itself - - ask us)