This is good advice, but it assumes there's a set of appropriately-difficult tasks to tackle, relative to your current ability level. Not so easy there's no growth triggered, not so hard you can't do them enough to get the trigger or can't attempt them without hurting yourself.
If you want to become physically strong, the default solution to this problem is to go lift weights. The idea is that you can challenge your muscles in the gym, build capacity to develop force, and then next time you need to use strength for real, it'll be easier. And this works, obviously. Professional athletes lift weights for good reason, and it pays off when they have more strength with which to push back the opposing lineman or whatever.
However, this isn't the only way to build strength, and done poorly it can have serious downsides.
The alternative is to just go do things that are hard. Not because they're hard, but because they're worth doing even though they're hard. A farmer doesn't need to lift iron so that lifting bales of hay is easier, he can just lift the hay -- and if it's hard, that will build the strength that makes it easier. If nothing else, this saves him a gym membership and time by doing his strength training on the job.
There's another more interesting advantage though, which is that the feedback loop is tighter. If you're trying to lasso a bull and your grip strength is what's failing, guess which muscles are going to get the impetus to grow? If you're trying to lift a big rock into the back of a truck, not only does each muscle get its own growth signal in proportion to its need, you get practice learning to use them. There is skill in coordinating muscle contractions into the right postures to use muscles to do real things. There's skill in knowing when to not use muscles. While I'm sure NFL coaches do a pretty good job of matching training to the field use, it's easy to overemphasize bicep curls because biceps are what you notice on people that "look strong", and now you've invoked Goodhart's law. I've had a woman I know tell me about guys at Jiu Jitsu who at first intimidated her with all their muscles, only for her to realize after rolling with them "Lol, those are gym muscles" — because these guys lacked the body awareness needed to actually use them effectively.
Figuring out what we're lifting for matters, because it determines what we lift and how we lift it. Even if we're just trying to impress people, it's hard to be more unimpressive than spending all our time building muscles that don't actually work when we need them.
It is said that meditation is strength training for the mind. Why are we strength training our minds again? What is it that this strength is for, and are we keeping this in mind when we choose what and how we lift?
Meditation has all the same considerations and more, because staying connected to what you actually need meditation for is itself a meditation skill. Arguably the most important.
Anxiety? Why not do CBT or something. Tackle that directly, with whichever skills prove appropriate, and concrete feedback about whether it's working. How anxious are you, and is it more or less than one perceptual timestep ago? What is correlating with these fluctuations, and what does this imply about how to solve the problem?
Or better yet, why do you care about anxiety in the first place? What are you anxious about anyway?
Oh, public speaking? You're worried that you're gonna do a bad job, and this is pulling your attention away from where it needs to be in order to do a good job? Why not tackle that directly, with whichever skills prove appropriate? Skills like holding attention on the topic of your speech. Like awareness of when your attention is drifting towards unhelpful things, and of why your attention keeps getting dragged where it does. And whether there might be a reason that has elided your awareness.
Whatever the goal, there's always the option of intentionally practicing mindfulness towards the specific things needed to succeed at your goal. Instead of getting your koans from meditation teachers, you can notice the koans that nature puts between us and our goals. If you're diving for lobsters you might think the limiting skill is grabbing them before they get away, but the limit might actually be in your perceptual ontology. Which one of these pieces of kelp — which is definitely kelp — is about to transform into a lobster?
To the extent that becomes limiting, lobster diving becomes a great way to build the skill of noticing the difference between our low level conclusions and the reality beneath. Would you notice if you were up against this limit? There's something to meditate on. Something that will keep your practice grounded in what you care about.
Without that, and without a skilled coach who can keep your efforts grounded for you, there's the risk of developing the meditation equivalent of "gym muscles". Of becoming someone who is breath aware, and who can let thoughts arise and pass "as thoughts" from a full lotus on the meditation cushion, but reverts to conflating their thoughts with reality under the pressure of thoughts that matter, and might even be right. Without any hard earned skills of coordinating thoughts in ways that stand up to pressure and serve to shape reality.
When the thought that arises is "Shit, I left the front door open. I don't know where my kid is", do you have to leave the meditative practice in order to act? Are you building skills for dealing with thoughts of the sort "We're on track to unacceptable outcomes" that don't require either giving up on meditation or giving up on reality?
This alternative is much harder. Try to do the thing you actually care about. Hone the skills of noticing exactly where you're going wrong. Develop awareness of where your perceptual ontology is failing you. And of where your awareness is failing you. Puzzles will multiply faster than the answers.
And as a result, things will stay interesting. It can be hard to stay motivated lifting weights up when they just end up back down in the same spot anyway. Or to focus on your breath when your breath isn't doing anything interesting, and you have to practice not-noticing the lack of a compelling answer to "Why am I doing this anyway? This is stupid".
It's a lot easier to exert yourself to maximum capacity when the connection is solid, and you know what you're fighting for. When there's a bull to restrain, or lobsters to catch. Or unacceptable outcomes on the horizon, and no clear way off the track.