When you're already fit and healthy, it's harder to make big, dramatic strides. You're already eating right. You have to work harder to tweak your diet. Your exercises have less of an impact now; you need to "shock" your body to make bigger strides in your fitness level.
It's the same way with speaking.
The more experienced you are as a speaker, the deeper and more refined your training becomes.
You're not starting from scratch, learning the basics of engaging the audience, moving your body effectively, making eye contact, telling stories, and all those things that are complicated and scary for beginners. When a beginner starts from nothing, he or she is going to make great strides in their progress. They're starting from zero and every breakthrough is a big step.
Not so with us veterans.
We don't make huge breakthroughs most of the time, so we have to focus on the subtle tweaks and adjustments that allow us to continue growing, continue mastering the art and science of speaking, continue creating more effective presentations, and continue building our connection and relationship with our audiences.
Don't think that your growth and progress are "over" because you've been doing this a long time. Look at your progress as small refinements with big impact.
When's the last time you introduced a new exercise or activity into one of your presentations?
When's the last time you thought up some new questions to ask your audience?
When's the last time you challenged yourself to try something scary, like singing or acting out a character or something that's new and different for you?
When's the last time you added some content that rocks the boat or might be considered controversial in your field?
If you're not doing these things, you're stagnating. And, like the fit person who gets lazy about exercise and diet because he's "already there," you lose your edge, you lose your fitness, you lose your muscle tone, you lose your conditioning.
Dig deeper to find that next level of mastery. Add new tricks to your bag. Complacency will make you weak and flabby as a speaker, just like it does to someone who's in shape and thinks they don't have any more work to do.
Get creative and stay fit!