How does this sentence sound to you?

There's no reason for "at one sitting".

Do you perhaps mean to say,

I walked, without once sitting, for no less than 3 hours from my house to her house yesterday.

OR

I walked, sitting only once, for no less than 3 hours from my house to her house yesterday.

JTT is right of course as usu. However it would help Imsak if you could define "at one sitting"

Feel free to go right ahead, Dale.

We generally only talk about "one sitting" in connection with activities that are performed while sitting, for fairly obvious reasons. Most often it is used about eating and drinking: I once ate three dozen oysters in one sitting; My mother once drank a bottle of gin in one sitting. Nobody that I have ever heard of sits while they are walking.

This will be based on one idea.

Con my very reaction

Or,

Is it 'in one sitting' or 'at one sitting'? Can it be used interchangably?

I came across this phrase somewhere before and looked up for the meaning in some dictionary and it tell :

if you do something at one sitting, you do it during one period of time without stopping; during a short period of time; all at once; in one go; in one gulp; in one breath; without stopping; without pausing; without resting; on end;

So I maybe mistakenly took it for granted that it might be able to be used in every situations. Now with all your explanations I am aware that it should be used only for the situation that we are doing things while sitting. Am I right?

Thank you to everyone.

So what did you actually mean to say, Imsak?

Im, see JTT immediately above

Maybe, with my poor English, I often failed to impart what I really want to say.

There is not much to it, i just want to tell that I misunderstood to the use of the phrase, but now all replys have made me more understanding.

Your meaning is clear. You walked 30 minutes without stopping. Say you did it without stopping instead of at one sitting. It just makes better sense.