It doesn't matter how much you do at one time, because even with little breaks, it all adds up. This morning, I woke up and ran 3.26 miles in 24:01, a 7:21 min/mile pace. I started off slow and eventually sped up to get down to that point. After working at school, I went home and ran 8.05 miles in 58:27, a 7:15 min/mile average. Now for this one, I was able to keep a decent pace without pushing too hard. Because of the hard work that I've been able to put in, I've grown accustomed to running a lot, and I am able to improve my body to push it to the next level because of my morning and evening runs. Afterwords, I went to the gym and did 22 pull ups, 52 push ups, 1:10 of cycling (19 miles worth) and 260 single calf raises as well as 110 vertical lunges/leg lifts for each leg.
By itself, it wouldn't have been a big deal, but putting all of it together, it worked my body hard.
Experience builds, and when working on endurance, it builds on top when you don't get the full amount of rest. To endure, you have to be put in the actual situation. Enduring means that you must be going through some sort of troubled times. During that time, the inadequacy of rest improves the endurance. Your body will adapt to whatever it goes through, and when it's dealing with not as much rest, it uses that opportunity to improve stamina/endurance.
Use every little time to make a difference, because just like a river running over a jagged rock, over time, the rock will become smooth, like a pebble.