When I tell people I've just completed a century ride (or a marathon) (or a triathlon) (or any athletic endeavor that took longer than a hour), most people offer their congratulations which I always happily accept. Then some humongous percentage of those people say "oh, I could never ride a century" (or "run a marathon" or "complete a triathlon") and then they say something to the effect of how special I must be.
C'mon people! Look at me. Look closely. I'm so far-the-hell from being physically special that it's scary. I'm almost fifty-freakin-five years old. I'm overweight by 15-20 pounds (and that's being generous). I'm constitutionally lazy and never exercised at all until my 30's, and not seriously until my late 40's. I have no upper body strength, a sore back and a painful knee. I smoked heavily for 23 years and now have exercise induced asthma. I don't sleep, don't eat right and drink too much wine.
Since I can cycle 100 miles or run 26.2 miles or complete an Olympic distance tri, I'm convinced that any vaguely healthy person can too. Don't tell me that you're slow, I wrote the book on that. Don't tell me that you've never run more than 5 miles, neither had I until I started training (for that matter, I hadn't walked that far either). Don't tell me you don't have time, because there are 24 hours each and every day and you can set aside one or two of those to train (you really don't need to spend all that time watching tv or reading or seeing the newest movie). Don't even tell me you're missing a limb because there are endurance athletes using prosthetics.
Be honest with yourself, if not with me. Next time you try to tell me or anyone else that "I could never do [fill in the blank]" just stop yourself, correct yourself, and say "I would never do [that thing]." You have other things in life you'd rather be doing, things that bring you more joy than being active outside. That's ok, not everyone wants to spend hour after hour running or cycling or swimming (or skating or dancing or skiing or whatever), following a schedule and building up endurance. Next time I tell you I did some athletic event that took me the entire day, just congratulate me, shake your head in wonder (or sorrow) and whisper to yourself that you'd never do that. Don't try to convince me that you can't.