Paid fit is worth it if you can afford it, is comprehensive enough and is convenient for you.
For myself I searched around on the internet and bike forums to get an idea. I only adjust the saddle height, aft/front position, these make a lot of difference to riding comfort on long rides, my longest distance is 40 miles and daily rides of 20 miles.
I do agree for short hops you don't need a bike fit, but if you are doing serious about biking long distances, then do go for a paid bike fit its probably worth it. Just check out the reviews of the place you do the bike fit. If possible ask if they could suggest an appropriate saddle for you (if the one you have is unsuitable). Bike fits should be in-depth and comprehensive otherwise there is no point.