They are all good and all will help improve each other's time. I have only done 5Ks but I have been increasing mileage and plan to run a half for the first time. I run almost half marathon distance (8-10 miles) just to train for my 5K pace.

I am sure if I stay hooked on the sport I will eventually do a full one and 5ks will seem boring. Hell, a 18-22 min 5k workout can get really boring quick.