I've been running for 5 years now and, based on my experience, I can say that when I lace up my shoes and go out I usually run setting 4 different paces, according to my purposes or the external conditions. Here is a table that recap them:
When I simply intend to enjoy the landscape or contemplate a particularly beautiful horizon, I choose the Koala mode. This is also the pace I prefer when I need to warm up, when I am not fit, when I have to recover after an injury or a long stop, or when the weather conditions are not favourable (either too cold or too hot/humid).
Class B is the most effective if I want to improve my stamina and my endurance. Some athletes consider 5'00" a jogging pace, but for my standards it falls into the category of training speed. Furthermore, running at this pace qualifies my performance as "anti-LDL cholesterol" measure (and pro HDL cholesterol run).
When I feel as fit as a young gazelle (unfortunately it doesn't happen every day...) and when I feel that I can improve a PB, I choose the class C pace. Here the weather conditions play an important role, and this is the reason why I love to run in September, when wind is not that strong and cold and the temperature is around 20° (I am talking about Mediterranean environment). Usually I run at this speed in a track or in an even, measurable path (3, 5, 10 or 21 km).
The Lizard pace is for interval training and sprinting sessions (400, 500, 800, 1000, 2000 meters). It is functional to improve class C medium-distance performances.