/Agree for above posts, but you can do intervals for much longer. Its all on what you have trained your body to withstand.
In training for marathons or triathlons, may people simply go for distance and neglect strength/speed training. A favorite of mine is ladder runs. Jog for 2 min, sprint for 2, 3 jog, 3 sprint, up to as much as you can withstand (usually 5-8) then ladder back down.
You get the speed and constitution. Its about getting your body used to the fatigued state.