Personal trainer Matt Hart replies: There are three variables to consider if you want to get fitter and you've considered them all, so well done! They are 'time' - how long you run for; 'intensity' - how fast you run; and 'frequency' - how often you run. You should be able to change any of these to get fitter and faster. Start by lengthening one of your weekly runs by half a mile every four weeks until you're running six miles. With the other session, focus on increasing the intensity and running faster. Break your three-mile run into mile sections and alternate between a normal pace, a fast mile and a recovery mile. Over time you could add another session per week, but perhaps cycling to give your joints a rest.