Just simple play will definitely not enough for improving!
Take a Chess Coach, who will analyze some of your games, and offer suggestions and solutions.
You can rely and follow, if you are A GOOD STUDENT!?
The really good books are very helpful! Read the David Bronstein! Alechin. Reti.
I would highly recommend reading numerous books. Since I don't know what your "real" level is yet since you only have 1 game under your belt at the time of this posting, it's hard to say which specific books would be best for you.
I would probably start off with working on your tactics, so books such as "Winning Chess Tactics", along with the tactics trainer on this site will help you out tremendously. Once you're good at recognizing and implementing tactical motifs such as skewers, pins, forks, discovered checks, windmills, etc...you'll want to move onto more theoretical / positional books such as "The Amateur's Mind" and "Reassess Your Chess" by Silman.
I would also recommend closely reviewing your lost matches with a chess engine that can tell you what you "should" have done, as well as how strong your move was that you made. I am on a Mac, so I use Shredder 11, but there's lots of great ones for PC, such as Fritz.
Good luck!
How is it possible for me to advance in middle game ability? Should I take coaching or read books, or just plain play? My opening is fine and I get into great positions, but then fumble it away in the middlegame.