Is this a run-on sentence?

[snowmanlover]

I started a story with this sentence. I don't think it's a run-on, but I wanted to make sure.

Thomas didn’t like doing errands, especially running to the grocery store, but if he hadn’t that day, he wouldn’t have seen the woman in the spice aisle.

[Umbre]

You may be better splitting this one up, as I don't feel that a semicolon is going to help the fact you have two interrupters in the sentence as it is.

Thomas didn’t like doing errands, especially running to the grocery store. But if he hadn’t that day, he wouldn’t have seen the woman in the spice aisle.

Or even :

Errands, particularly the grocery store, were not an activity Thomas looked forward to; but today the usually unpleasant venture was fruitful. It was on aisle 9, near the cakes and spices that he saw her.

[BluSyndrome]

I was about to suggest the same thing as Umbre did, but obviously I didn't because well it's already suggested. There are lots of ways to go about this sentence, and I think what you have currently is the plainest of them. Add on to it! Change it into two sentences, or three, or four! I think you have a really good start and with a little tweaking it should be awesome.