Shall I try?
A run-on, according to that arbiter of punctuation, wikipedia, (there is nothing about this in my "Essential Guide to Grammar" book you'll be pleased to know) is where two separate sentences
are joined inappropriately. (Presumably they're thought to be
run on as the first sentence should have ended with a full stop before the other started, but instead it ran on into the other.)
If you have proper punctuation (eg a semi-colon) or a proper conjunction/linking word (eg and or but) then you can join as many sentences as you want, without being a run-on, though obviously after a while it begins to look ungainly.
So the dread comma splice "He walks forward, she turns" is by this definition a run-on. But sentences like "He walks forward
as she turns" or "He walks forwards
; she turns" or "He walks forward
so she turns" aren't. (Whether they're any good is another question, of course.)
To my mind there are no run-ons in those two sentences you quote, which is perhaps why you're getting confused. However, the comma after "task" is a problem we've raised on Chrons before. Strictly, you should hive off "as she returned to her task" into its own sub-clause:
He crouched down and, as she returned to her task, he snapped a few shots.
but that can read as unnecessarily stilted, so I certainly wouldn't insist on it. (I spend inordinate amounts of time putting commas in constructions like that then taking them out again.)
Basically, Mouse, if you want my advice, just keep writing as you are.
EDIT: Pish. spent so much time on this everyone and his/her spouse has answered again.