Question about endurance + speed

I just started to take running seriously as a part of my lifestyle (both fitness and sports) I am wondering about any good ways for me to build up my endurance and fitness while at the same time my running speed.

So far I have just been focusing on getting up my fitness by running a bit further each day. I also mix in sprints as well as drills using stairs.

Lastly I take the time to work on my running form, but it sure is hard to focus on that when your legs are dying ;)

any suggestions?
 
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