Why are people so worried about over exerting themselves in workouts?

Meh, you’re not skilled enough to differentiate between 90% and 95% so it doesn’t really matter what you do.

you know what I mean though -_- or I can give another example: 85 ish percent vs 95 ish percent

Are you talking about the classic 20min tempo? According to Daniels, if you run them at threshold (LT2) pace, then that’s a little slower than 1/2 marathon pace, what can be run for an hour until exhaustion. So if you are doing one of those per week, and a track workout, and a long run, then the rest of the runs at Daniels’ easy pace (7:06-7:50 for a 17:00 athlete) all add up to progressive work, that can lead to overtraining if running it all too fast.

Okay here’s a semi-serious answer.

it’s like sticking your hand into, or over, a fire. How long can you hold it there, if you had a gun to your head? That’s your race pace.

Now you get to practice it everyday. Maybe you hover over it by a few inches, and can withstand a minute or so. Or you stick it in real quick, and pull it back out soon afterwards. It’s a balance of intensity vs duration.

Overdo either of them, and you’ll burn yourself.

If you aren't running sub 4 in the mile you should not be typing this out...

Because a 100% effort doesn't actually provide as good a training effect as a 90% effort, especially when placed in the context of an overall training week/month, etc.

If it did, you'd see people racing every 3 days (year-round), and they'd achieve better results than those that have a set racing season and/or race sparingly.

But given that that isn't what happens - and anyone who tries to do this quickly finds out why you shouldn't - most successful runners have realised that they should leave their all-out efforts for race day through a process of elimination.

Also, if your tempo is 95% effort, you likely aren't training the thing you're intending to.

When you run a tempo, you're trying to utilise and develop your aerobic system. If you run too fast, you'll rely too heavily on your anaerobic system (which has a very limited capacity), thus defeating the purpose of the workout.

I'll use an analogy, let's say you're trying to strengthen your glutes by doing squats. Now, you could lift a much heavier weight if you recruited a heap of other muscles in your back, shoulders, quads and wherever else - while employing really bad technique - but you wouldn't achieve the objective of strengthening your glutes, and you likely wouldn't actually get stronger (more likely just eventually injured).

You sort of made the opposite point. A 15:30 5k is 5 minute pace. You could do one again in 3 days but you could instead run 4xmile in 4:55. The send option is more work at a faster time.

Let me also use an analogy that includes both squatting and fire. Assume there’s a flame burning on the ground that can scorch your rear in the hole position. You get the rest of the analogy.

Your chest and legs are two different body parts. Where is this anaerobic system as opposed to the aerobic system? I only have one heart and set of lungs. Maybe you have two.

By running workouts for specific purposes at specific efforts that typically range from moderately difficult to very difficult, but virtually never include all-out efforts.

You train to target specific stimulus to provide a specific adaptation. Most adaptations required for improved fitness happen at efforts much lower than race paces. By sticking to the lowest effort required to get to desired adaptation, you then in term minimize fatigue and recovery time needed, allowing you to do more hard sessions over the same given timeframe as opposed to if you went harder than need be.(in the latter, not only do you now need more recovery time, you haven’t achieved any more fitness gain because the stimulus is the same as it would have been at a little less effort)

When you finish your adolescence and become an adult in a few years, you might begin to understand why easy training is important.

We got a hanger-on over here

Now that I'm older and wiser I've realized that I probably spent some of my best efforts in practice. I'd run a killer session, coaches and friends would hype me up saying I'm going to run such and such time. Come race day, I miss the mark by a lot. It's totally possible some of those were just bad races, but I know that I gave my best effort on a random Tuesday workout a few times instead of the race on Saturday.

This is correct.

Let me give some over-simplified examples.

Let's assume the training plan calls for a 20 minute tempo run. Done properly, these runners get 20 minutes at their optimal HR to build VO2 Max. (Okay, they don't get it for the first couple of minutes because they are still warming up, but bare with me. This is a SIMPLE example.)

Now, let's assume they run too fast. They get 15 minutes aerobic exercise and 5 minutes anaerobic exercise. They have gotten only 15 minutes of exercise at the optimal level for that particular workout. (In reality, you don't switch from 100% aerobic to 100% anaerobic, but you get the idea, right? That is, running too hard REDUCES the volume of the workout of the desired energy system.)

The other problem is that now the body has to recover both the aerobic AND the anaerobic systems before the next hard workout. There's a chance that even one easy day is not enough to optimize the quality of the next hard workout.

Ask the Tufts University lacrosse players and their navy seal trainer.

A lot of high school and college athletes race their workouts. They run their reps all pretty hard and then as fast as they can on the last rep or two. And they do that every workout, every week. This is basically racing twice per week, which is too much. You can kind of get away with this if you're not doing big workouts, but the more volume and bigger workouts you do it becomes far too much to be doing 6+ miles of intervals or 'tempo' work as fast as you can twice per week. It's too much of a beat down on your body to fully recover in 2-3 days before your next race, I mean workout.

It's me, hi, I'm the problem, it's me, it's the 1990s!