I would be telling myself: How can I commit to a lifelong project like children? How can I decide what career path to choose, when there are so many options and I can only pick one? How can I work out, when its basically signing up to humiliate yourself? How can I take these _risks_?
In my experience, anxiety pretends that it's useful. It feels motivational -- like you are motivated to work out, because of your anxiety about being frail.
But contrary to how it feels, anxiety can have the opposite effect. You want it too much, that becomes dangerous, because you're afraid of trying and failing.
If that sounds familiar at all, my advice (since all HN comments have to have advice, right?) is to distance yourself from those goals and aspirations. Perform the Stoic exercise of meditating on the worst-case scenario. Let yourself make mistakes and look stupid. Just do it because you should, and damn the outcome.