Go a slower pace to start with for a decent bit of time, or do high intensity for a shorter length of time. Or you could just find your own rhythm. But I usually just crack on some good music and get to goin at a pace that doesn't make me want to shoot myself and sustain that for a long time, before I know it I'm at 40min and the runs over. 40min is my sweet spot to where I feel accomplished no matter the pace I was going and also have worked up a good sweat.