Speedwork pushes you out of your comfort zone - and improves fitness - faster than any other workout," says running coach Nick Anderson (runningwithus.com)."You recruit muscle tissue that otherwise lies dormant and you train your body to become efficient at faster paces."
Unfortunately, running tracks can often be unavailable, inconvenient or intimidating to many runners. So here are four other ways to get your fast fix.
On the Road
Training on tarmac simulates the conditions you'll face during road races. The impact can take its toll, "but that's what you want - up to a point - as this makes you stronger", says running coach Andrew Kastor.
The workout: Do 800m reps at goal 5K pace. Take 400m jog recoveries, which are "long enough to clear lactic acid from your muscles, but short enough to keep things challenging", says Anderson.
Beginners should do three or four reps; advanced runners can do up to 10. Measure the distance using GPS or, if you listen to music, try hitting 5K pace during one song and jogging during the next. Repeat the pattern, aiming for six to 12 songs in total.
On the Trail
"Trails are easier on your legs and they improve balance and coordination," says Nancy Hobbs, co-author of The Ultimate Guide to Trail Running (£15.95, Falcon Press). Be sure to watch your step on unstable terrain, though.
The workout: Try trail fartleks. Running fast uphill builds strength; running fast downhill increases turnover. Jog for 10 minutes, then go hard (about 10K pace) for 30 seconds, easy for 30, hard for 60, easy for 30, hard for 90, easy for 30. That's one set. Perform three to 10 sets in total.
On the Treadmill
"The forgiving surface of the treadmill allows you to recover from speedwork quicker," says Kastor. On the flip side, the belt's movement alters how you use your hamstrings, which could lead to muscle weaknesses or imbalances, and then injury, in the long term.
The workout: A progression run gradually gets you working hard. Do a 10-minute warm-up jog at about 6.5km/h. Then up the speed by 0.2 or 0.3 km/h every minute for five to 10 minutes.
Take a one-minute jog or walk recovery, then begin the next progression at the speed at which you left off. Again, increase by 0.2-0.3 km/h every minute for five to 10 minutes, jog for two minutes, and start again where you left off. Cool down with a five-minute jog.
Off your Feet
Doing speedwork on a bike, cross-trainer, step machine or in the pool delivers a boost without impact. "If you're injured, this is the best way to maintain your fitness," says Anderson.
The workout: After a 10-minute warm-up on the machine of your choice, go hard for 30 seconds, one minute, two minutes, four minutes, two minutes, one minute, 30 seconds.
After each hard segment, go easy for the same length of time. Beginners should stick to one set, advanced runners can do up to three (go easy for three minutes between sets).
Fine Tune your Training
Make the following minor adjustments to your speedwork to meet your running and racing goals faster:
Goal: Improve speed; mile or 5K race
Speed tweaks: Run shorter, faster repeats (one to two minutes or 200-800m) with long recovery intervals (three to five minutes).
Goal: 10K or half marathon
Speed tweaks: To develop speed and endurance, split your speedwork days evenly between short repeats with long recoveries and long repeats with short recoveries.
Goal: More endurance; marathon
Speed tweaks: Run longer, slower reps (four to 10 minutes or 1000-2000m with short recoveries).