For a number of years, I worked with another coach who regularly expressed frustration that athletes he was coaching weren't willing to accept that they had simply reached their genetic limits and weren't going to get any better.
"I wish I had a tool that would show people what their maximum possible performance will be based on their VO2 Max. Then they'll stop complaining to me that they're not getting the results they want and stop questioning whether the training I'm prescribing is the best for them!" was his regular refrain.
It'll come as no surprise that I deeply disagree with his view on this. I don't believe that any athlete I have ever worked with has come even close to bumping their head on their "genetic ceiling". That's not because the training wasn't right for each individual, nor was it that they didn't try hard. Even the 3 masters/age-group world champions were nowhere near all they could be, if for no other reason than the fact that they had lives outside their sport.
I'd go a step further and say that I don't believe that the best in the world at any endurance sport are even close to their genetic limits. In fact, I don't think that genetic limits are a factor at all (cue the "boos" from the sports scientists in the crowd).
I don't really want to go into the reasons why they're not there, but rather give you a few actionable items that will help you get more from your athletic self.
1. Train consistently over a period of years.
Often when I'm approached about coaching, the goal is a fairly short term one - 3 to 6 months. In that situation, I'll help the athlete to get the best result possible, but for the real jaw-dropping results come from a period of years in which you do consistent targeted work.
2. Work on your efficiency.
Some of the marathon world records in the past were set by runners with woefully low VO2 Max numbers. Yet they still ran faster than those more genetically gifted - VO2 Max appears to be pretty much genetically limited, thus my erstwhile colleague's desire to use it as an excuse.
The reason these guys were faster is that they developed an incredible economy of effort. Put another way, they were able to run fast whilst using less energy. You might even say that they have overcome the genetic limits of their VO2 Max.
The sort of thing you can do to improve your efficiency, for example, is to remove unnecessary habits like clenching fists when running, fighting the bike with your arms when riding or simply cleaning up your swim stroke altogether.
Having an expert provide feedback on your form when doing your sport can be extremely useful in picking up those things that are costing you needless effort, but you could do something similar simply by getting a friend to video you in action, then figuring out what parts of your performance just "look wrong" and consciously modifying these for a few weeks or months before repeating the video process.
3. Maintain your body for the long term.
Endurance athletes are generally awful when it comes to looking after their bodies. We simply go out and train despite the niggles (which then become injuries), because we "have to get the miles in". Over time, these small injuries become long-term adaptations in our bodies and those changes take a lot of focused work to reverse.
Incorporating a daily self-maintenance routine into your training is key to catching these small things before they become big things. Often that small niggle is just a bit of an adhesion between a few muscle fibres or between the connective tissue and the muscle, which would be resolved by a bit of self-massage, foam rolling or similar. Instead, we go training, force our wonderful bodies to work around the problem and in time the tight spot in a calf becomes a sore hip!
4. Steer clear of workouts to failure.
This is a big one for me. As I've written before, workouts that are impossible (or even nearly impossible) to complete serve as a threat signal to your central nervous system, whose function is to stop you killing yourself. Often this can look like you have reached your genetic limits.
While you can often override fatigue and even pain, this will only be for so long. Eventually, your CNS will simply shut you down. And other than being dead drunk, drugged up or under anaesthetic, there is no way to override your CNS.
However, if you stay away from failure-level workouts, you can slowly but surely con your CNS into allowing you to do more.
You truly are a wonderful machine - not just your body, but you as a whole. If you are willing to be both patient and mindful with your training, you'll be awestruck by what you're actually able to do.