Every race must be broken up into manageable portions, particularly half marathons and marathons. We call this technique mentally "chunking" a race, and while chunking is not a physical skill, it is just as important. But how can you actually practice chunking?
Break your longest training sessions into different portions and make notes to yourself, advising what your focus should be within each chunk. For example, if you have a 20-mile marathon prep run, use the first eight miles as your warm up with mental self-talk, such as, "Easy, relaxed to begin, totally dialed back." With the meat of the run, miles eight to 16 transition to thoughts such as, "Forward, keep cadence high, maintain tempo." Lastly, as you enter the final three to four miles (and are likely hurting), tell yourself, "Stay focused, light, relaxed and strong all the way home." While these mantras may seem silly, they will go a long way toward breaking up your training runs and races into manageable pieces.
You can also break training into different venues within the same workout. Chunking can be as simple as starting a session on the track, running the middle portion of the workout on the roads and finishing in a park. Rather than the typical same-venue-every-time routine, split your longest and toughest sessions into different geographical locales—perhaps based on different paces within the workout.