When I'm writing the zero draft--which is my phrase for all new stuff that I am figuring out mostly as I go--I find it much harder to pay attention to length and pacing. This might be, partially, an artifact of me counting words in each writing session. If I'm tired, or losing my concentration, and I haven't put down as many words as I'd like, I will sometimes add in more wordy description, or let the dialogue go on farther than I would if I were being more careful. That doesn't always end up being a bad thing. If I describe a room, and in a later draft decide that paragraph is too long, sometimes I can use the rest of that description in another place where it's more appropriate. Sometimes, letting the dialogue ramble a bit brings me surprising new insights, even if I cut the bits that aren't directly to the point.
If I'm going to add another hundred words of description to my day's work just to make wordcount, I am clearly not thinking about how that will affect pacing, except for stray thoughts like, "I'll fix that later." However, I don't think zero draft is the place to worry too much about pacing. Sure, I don't want the story to be bogged down in ten pages of description. I mean pacing in the more delicate sense, after I've gotten to the point when removal of one paragraph makes the plot snap into place. I can't know if that paragraph is needed or not until I have a whole draft. So, back to the beginning: don't worry, keep writing.