There is a lot of debate about this topic in the running world still. The "180 as optimum" was taken after viewing footage of elite marathoners mid race, and counting steps. There has been some confirmation that some marathoners train at that cadence as well, but there have been just as many that don't fall into that range.
What I've found as a coach, is that the vast majority of runners will self select the cadence and strike pattern that best suits them. I also think it's unreasonable to expect someone to have the same cadence while running a 10 minute mile as they do running a 7 minute mile. That generally means that you're either artificially chopping your slow pace stride short, or that you're over striding in your 7 minute mile.
Just go run. Look at your shoes when they start to get worn, and see if you have any unusual wear patterns. If so, then you may need to adjust your stride and/or type of shoe that you wear. If not, don't worry too much about it. Your cadence will increase naturally as you run faster, and your pace will get faster for the same cadence as you get more efficient.
What you should do, if you are intending to race, is have days of dedicated speed work, as well as short fartlek/stride intervals on other selected running days. There are quite a few questions on here about adding speedwork to running.