If you have Apache2 in /usr/local you (or someone else) installed Apache2 there.
There is an Apache 1.3.??? server that comes with
os x , and is turned on by "Personal Web Sharing" or
something like that in System Preference > Sharing.
So, you can't run both of them, unless you override
the listen port on one of them (the default is 80)
Also, it might have simply been already running!
Did you look for it? It would be several process named httpd in the activity monitor in Applications > Utilities
Next time, try restart instead of start. Then if
it complains, it's probably the first problem.