If you haven't ran for ages, you probably have a small heart so don't push it too hard, start with 1.5 miles, get the timing down to around 10-11 minutes or so. Then start increasing the distance from there linearly with time. If lets say your starting time is 20 mins, target a 1 minute reduction every week. That's probably as fast as your heart muscles will grow. Also, I'm a short distance high speed runner, burns as much calories as long distance but the timing ratios are different so get the 6mile timings from your cousin.
You can tell if your heart can't keep up if u get pins n needles in your arms and feel light headed, if u reach that state, stop running. Walk the rest of the distance to cool down.
There's another factor of type 1 (slow twitch) muscles being under developed in your legs, give that time to grow too. You'll get 'burning' heavy legs due to lactic acid buildup..... and don't forget full stretching exercises before and after to avoid injury. Over hydrating over the 4 hours before the run is a plus.