Yeah consistency and good form. If you lose form, you can injure yourself pretty good. I ran cross country and track in high school.
Make sure for long runs you are properly hydrated throughout the day. The frequent urinating might be a hassle but you'll need all that water if you're going to sweat a lot. But you probably already know that.
For the long runs (10K +), I start off almost at a walking pace and build myself up from there. It's a pretty natural transition and the walking to running happens in well under two hundred meters anyhow (probably closer to a hundred meters). The smooth transition helps me run a lot longer than if I start with a completely running start. Obviously in events like the 5K, starting in a walk is not an option (you'd lose too much ground in the beginning) but for longer runs I've used it to great effect. I find myself running much faster and for longer later on in the run when I do this.
Stretching is obviously important. Electrolytes as well.
If you're prone to over heating, then pouring some water on your head and shirt helps. I've found it to be much more effective than letting sweat do it all by itself. The added weight is negated by the advantage of a lower body temperature.
Swimming... it's technique heavy. You can be very fit but if you have poor form, you'll bleed energy like nothing else. Form counts here, probably much more so than running.
Can't tell you much about cycling though.
Sadly I am no longer in any shape to run events like marathons.
Knees won't take it anymore. Long story.