I used to run Cross-Country many years ago. My 5K times were in the Low to Mid 16s as I recall. But, I can't really think of any "pointers" other than to run for a set period of time every single day, rain-or-shine, hot-or-cold.
Start with whatever you can run now; I'd imagine if you're running a 19 5K, running for 30 minutes isn't too hard. Start with what you can do now and every so often add a couple more minutes to it. This is where it takes motivation and honesty with yourself. Only you know what you can really do now and only you know how hard you can push yourself. There is no set formula for this type of thing.
