Something I do when I can remember, is when I am running 'steadily', ie I can just about hold a conversation but heavy breathing... instead of breathing from empty lungs to half full/two thirds full, I breath from half full to completely full... this doesn't mean you hyperventilate, but it does help open your lungs, and trains the muscles around your ribcage on the extremes of their movement.

Otherwise, when you ask them to open fully, they won't know what it's like to do that, and invariably will be the first things to fail.

I dont think it gets easier as steepler said. Your aims and goals get higher, therefore trying to reach them is harder.

Agreed ...

100% effort will always feel like 100% effort.
But you'll probably going faster and further as a result of that effort.

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GONE RUNNING

you have been given excellent advice. To be honest, 3 miles in 28 minutes for a new runner is good. But to get from that to 20 minutes will take a lot of time and training. When I first started running, I could do 3 miles in about 23 minutes. It took me a year and a half (building up to half marathon distances and running 30 miles a week) to get my 3 mile pb down to 19 minutes 30 seconds. I don't actually think I could run it that much faster. I think it is more important to build distance and stamina if you plan to run a marathon and want a respectable time. My average 5km (3.2 miles) splits for the last half marathon I ran were 23 minutes, which isn't actually that much faster than my top speed for a one off 5km.