A lot of not getting tired over a distance is actually technique and pacing, not fitness per se. One thing new riders often do is use too high a gear and that wears you out very fast. Try to press more lightly on the pedals but move your legs a bit faster to go the same speed by putting the bike in a lower gear. And don't try to go fast - go slowly enough so you can easily talk while you are riding. You didn't mention if it is hilly where you ride but if it is you need to make sure the gears on your bike are low enough so you can make it up the hills without getting out of breath. Start slowly and you will get faster very quickly as your technique and fitness improve.