You should be offended.
This person has judged you by your appearance and showed you disrespect inasmuch that they figure you aren't serious based on what they see.
It is typical behavior for the time. Unless you look like Eliud (and don't get me wrong...I love Eliud), or are 6'3" or more, many people are not going to respect ability.
I would take the slight and pour it into your training. It may not improve the way they act, but you will feel better.
And the first time your commenter complains about an ache or pain or the flu or something unhealthy, you can look at them and say...ahh...i feel good...now YOU can see why I run. Lol.
Oh,and never discuss running with this person again. They don't understand it.
I run for numerous reasons. The reason I run is for total health, both physically and mentally. It clearly shows. Most people my age where I live are overweight and unfit. I am trim and fit.
I had one person make an unsolicited comment to me as to why I run. They said "You run because you need to get your heart rate up". I'm offended by this remark. Would you be?