In app.js, let's create an array of run objects, I am storing the date as a string on purpose also, it's important that this is an array of objects, to work with D3). Here's what your app.js code should look like so far:

var WIDTH = 800;
var HEIGHT = 600;

var runs = [
    {
        id: 1,
        date: 'October 1, 2017 at 4:00PM',
        distance: 5.2
    },
    {
        id: 2,
        date: 'October 2, 2017 at 5:00PM',
        distance: 7.0725
    },
    {
        id: 3,
        date: 'October 3, 2017 at 6:00PM',
        distance: 8.7
    }
];

d3.select('svg')
    .style('width', WIDTH)
    .style('height', HEIGHT);