Those 2 muscle types are slow twitch and fast twitch. As yt says, the fast twitch is for weight lifters and it tires quickly, and the slow twitch is for endurance.
The secret to distance running is to build up your base of endurance. If you are not having trouble with your oxygen intake, then you are not trying to run too fast but you need to keep pushing your limits.
Go as far as you can on one day, then the next day go about 1/2 that far. That is your recovery day. On the third daym run about 3/4 as far as you did on day one, then on day 4 go as far as you can again and keep repeating that cycle.
Stay within your comfort zone and relax. Don't expect to see results the first week or two, but over the summer, you will have your endurance base built up to perhaps twice what it is now.
Oh, another thing, if your feer or legs start hurting, buy better shoes.