I remember a day a few years back when I was talking about riding my bike to work. I was standing around with two sedentary, overweight coworkers. When I disclosed how long it took me to bike the 7 miles, one of them responded with, "I could do that." I was dumbfounded. It's possible that he could do that, because I am not a speed-demon on the bike at all. He didn't, though. I don't even think he walked the mile to the train.
I find that many people who begin doing active things will encounter people like my coworker. People who aren't doing any activity at all, but will still find something to say to make your effort seem too little, too slow, and/or bound to fail. They may talk about the time they tried to lose weight and failed. Or they'll talk about their fitness exploits like they're no big deal, even though they're pointedly better than yours. I call these people naysayers, but the internet calls them haters. I really like the definition shown below.
I didn't know what to say to my coworker that day, and I'm still not sure what I'd say to him if given the chance again. I think the proper response is to invite the naysayer to join you in your next training run, 5K, bike ride or Zumba session. If they join you, great. If not, at least they're less likely to mock your efforts in the future.