I have always pushed myself hard on the bike I think my secret to speed was the long rides I used to put in at the weekends. 100-140 miles.
I say riding only 150 miles a week but that was toward the end of my riding days.
I have literally 1000's of miles in my legs and sometimes when I was in my 20's I would log 300-350 miles a week.
Also I rode a lot of hills that is good for building muscle and the heart gets to maximum easily.

I certainly benefited from riding the hills.