All due respect, I don’t quite agree with suggestion of going after speed first. As you start out a jogging program, your body will go through series of demanding changes. For every physical activity, it requires oxygen to perform. As the stress, in this case, speed, increases; so does the demand of oxygen. With the slight increase in running speed, the demand of extra oxygen shoots up dramatically (some may say it increases by doubling and tripling and cubing and so on). As you keep exercising at the same level for awhile, your body’s ability to retain more oxygen improves. In other words, because your oxygen retention ability is higher, now you can run at faster speed with same “effort”. Speed will be gained by improving your oxygen uptake and you cannot force it to improve. Well, I’ll take it back. You can force it to some extent but you will quickly hit the plateau. Body’s ability to retain more oxygen is best gained by increased capillaries in the working muscles. And it is best performed with long continuous running. You cannot run long by running too fast because you will quickly get into anaerobic state and the exercise will have to stop prematurely.
The best advice is to alternate the length of your runs by going one day short, say 20 minutes, and long the next day long, say 45 minutes. By the time you hit over an hour, you will be running faster without trying. Initially it is a good idea to go out on a trail or something after a certain length of time, say 15 minutes, and turn around and come back in the same time—15 minutes. This way you know you are not overtaxing your system. Once every other week or so, hop on the same course and run out same length of time (say 30 minutes and turn around and come back for an hour’s run). After a couple of month, you’ll notice you’re running further and further within the same length of time without forcing the pace. It is because what once was anaerobic speed, with the increased oxygen uptake level, now has become aerobic.