I want to get into bike riding?

Instead of buying new equipment, try to get on a bike and ride. The only way to do so is to ride. Step out of your house, jump on and start peddling. Very often I don't even plan where I'm going to ride.

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Why are you repeating youself?

Buy a cheap bike you will then see if you enjoy it?

Megan You buy a buy based upon where and how you want to ride. Based upon what you have written, you got the wrong bike, and the two you are thinking of are jut slightly better. UNLESS you intend on riding mountain trails the mountain bike is third best choice. It sounds as if you will be riding street, roads, trails and bike paths, That being the case, a road bike, either a racing bike (like your dad's) or a hybrid is the much better choice. Your existing bike if made for short, flat slow rides. For the long, fast group rides, over varied terrains, you need gears. Soccerref

To build endurance you need to add more distance every couple of times and you need to push yourself harder. At first you will get sore muscles if you are not used to it so don't go mad, take it easy and build it up. Flat ground with the occasional hill is a good place to start. Aim to ride most days if you are not too sore (if you are sore maybe a few easy rides around the block or something) about 3 miles would be a good start. Ride out 1.5 and then ride back. Pick a good route before you set off and go for it. Then you might add a few miles on into a week so your doing a little bit more each day. Change routes, change gears and Vary your speed, slow, fast, slow, fast. Eventually aim for 15 miles per day, mixed (flat and hills) flat is for recovery. Learn how to use your gears properly and wear a helmet always!!