Every long distance runner knows about pacing. It's a way to set a rhythm or get into a grove that allows you to run for a long time without using up your stores of energy. It might be necessary to sprint from time to time, but returning to the pace can allow you to relax a bit and even regain some of your energy.
The same is true in Aikido, though it can be harder to find and keep a good pace. Most Aikido classes are an hour long. Somehow, you have to find the energy to sit calmly and listen to your instructor, then practice techniques until the next break. It's hard to listen if you are huffing and puffing from exertion. Stamina varies greatly from person to person, and even in a particular person given time of day and what went on before practice. Only you can judge what the right pace is for you. With time and practice, you'll get a feeling for what the right pace is. With the right partice partner, it's possible to get into a pattern of practice that you can sustain for longer than you might think.
Another time to consider pacing is in randori (multi-person attack) situations. If you watch a beginner defend themselves against more than one attacker, you'll often see them wear out very quickly. There is a HUGE energy outlay in responding to more than one person at the same time. Then watch an advanced student. Notice how calm and relaxed they seem? It looks like they could go on doing this forever. While it's true that stamina improves over the years as you practice and a younger person tends to have more energy than an older one, there is more going on that just that. A skilled Aikidoka learns how to set the pace of randori, rather than letting his attackers determine it for him.
There are a few tricks to making this happen. The first is to use distance. Throw them far away from you. Move around the mat to create more distance. More distance requires more time to reach you. This increase in time causes the over all pace of the situation to shift to being slower. Hold downs can also affect the pace of randori. Normally, hold downs are not used in multi-person situations because while you are holding one, another attacks. To use hold downs for pacing, extend a bit of energy (Ki) into the person you just threw. Just enough to delay him (or her) getting to his feet again. This is a subtle thing, but again, it causes the pacing the overall engagement to shift slightly slower. Slowing down your technique can also help. Instead of reacting with blinding speed to a lightning fast shomenuchi, shift that energy into a slower, controlled technique. In general, other assailants will not attack while you are in the process of doing a technique (naturally, this is not always true). Slowing your throws slows down the pace of the randori. Finally, make sure you stay in control of your breathing. Breath is vital in a face paced, violent situation like randori. Breath deeply. Use your breath as part of the throw (Ki-ai).
Going All Out
Learning to pace your practice or techniques will allow you conserve energy and last longer, but you need to be aware of a hidden trap as well. Too much control can reduce your commitment. If you learn to hold back to save energy, it may become a habit such that you can't utilize the full power of aikido. So, just as it is important to pace yourself, it's also important to allow yourself times when you don't hold back. You may need a more experienced training partner and find an appropriate time (after class, for example), but give it a try. Going flat out will improve your stamina, speed, and timing. It's always easier to do things slow, but you'd like to be able to do things fast, too.
Pacing During a Test
Aikido tests are designed to stress the student. The testing committee wants to see what you can do under a variety of situations (appropriate to the level you are testing for). They will often vary your test partner to see how you react to someone big, small, fast, etc. As you advance through the ranks, the tests tend to get longer (and you don't get younger). As such, it's important to apply pacing to the test situation. Many of the randori tricks mentioned above can be applied during a test. You can control the pace of your test partner using them. Many test techniques will require you to demonstrate the associated hold down (or pin). Once you've executed the hold down, take a deep breath or two. You are in full control at that moment and it is a brief opportunity to relax and recoup.
That said, it is often the case that the test committee wants to see how you will perform when you are exhausted. They will deliberately run you to the point where you lose your breath, where your whole body seems to weight double or triple, till you can't even raise your arms for the next attack. Training in an all out mode is one way to prepare of this, but this is also where your spirit begins to show. Once you've gone past the point of exhaustion, you'll need to dig very deep to find a core of energy that remains deep inside you. Take a moment (if possible) to re-center yourself. Draw a deep breath into your center (Hara). You can response slower, but keep going. One source of encouragement is that it's almost over. You just need to get through one, two, or three more techniques.