... the highest gear suddenly feels like the middle gear.
... you can literally trot up 5 long flights of stairs to your office and your heart doesn't feel like it's in v-fib (like it used to) when you reach the top.
... the quads are showing! the quads are showing! the quads are showing!
... during your colonoscopy, the gastroenterologist remarks that your resting heart rate as recorded on the monitor seems rather slow at 60 bpm, and you smile because you know that's about 15 beats higher than normal.
... you consider maybe buying a pair of slim-fitting capri bike pants for the coming year to replace the now-too-big baggy pants you wore last year.
I was going to add "most of your wardrobe is now too big", but a slimmer shape isn't a reliable indication of actual fitness.
When 30 minutes of exercise no longer feels like two hours.