Watching other players, especially pros on tv or the internet is a great way to learn tennis by yourself.
Playing against EVERYONE you possibly can is also very beneficial.
1. You get more time hitting.
2. You learn about other people's strengths and weaknesses.
3. You can translate their strengths into yours (like copying technique)
4. You can improve your weaknesses by seeing theirs (like if you guys have similar weaknesses)
One thing that helps a lot when you are learning, is to be extremely in shape. What I have noticed is that the ones that are the most fit succeed. Plus you can win matches without ever having to hit one proper shot by just out-enduring your opponent.
Coaching is extremely beneficial however if you can afford. What I have done, and what I recommend for all players, is to get just enough coaching in order for you to learn all the proper techniques for all the strokes and thats it. A lot of players continue to get coaching from the same instructor even after they have learned the techniques. So after you learn the techniques, you continue to try to master those by hitting with other players. And then every few weeks or so you can get a coach for a very short time just to make sure you are hitting correctly.
Oh, one last thing: Consistency IS THE KEY! You might be playing against not so great players for now, but when you start going up the ladder, consistency is a requirement basically. If you cannot stay in rallies, good players will see this and just get the ball back waiting for you to make mistakes.
I'm sorry, but you're wrong about the whole "being in shape will make you win without having to use proper technique". Being in shape and having endurance is vital in tennis, but that will not make you win. If you **** around thinking your stamina alone will make you succeed, the other player will pull some nasty shots on you. Proper/Focused technique and gameplay at ALL times. On the other hand, if you mean the fittest player can MENTALLY tire out the other player, that is 100% correct. Devastate your opponent mentally, and the game is yours.
Another word advice to Topic Creator, practice at least 30 minutes hitting against a wall on a daily basis. That you never get "rusty" and are consistent with your form. If thats not possible, at the very least practice your technique in front of the mirror.