Re: No coach or practice partner for three weeks.. what do I do?
This has the scent of a Xristos post, but let's assume you're on the up and up.
You need to have more than one practice partner. Depending on where you live and the quantity of viable available partners, this may be difficult. Don't be afraid to seek out people of different ages who may be of a similar level. Of course, you usually want to seek out people that are better than you because this helps you improve faster. It's also ok to occasionally play with people that may be a bit below your level depending on the type of drills you're doing. If better players see you doing these kinds of favors for others they may be more inclined to hit with you.
The main point of having multiple practice partners is to give yourself more of a variety to the types of shots you will see. Some will hit with more spin, others with more pace, maybe you can find a lefty or two to experience the difference in the way their balls come at you. Plus, the more partners you have, the easier it will be to always find someone that is available. It's good to have a favorite practice partner, but if you only practice exclusively with one person you may find it difficult to cope with the differences in the games of your opponents when it comes to tournament time.
I don't recommend the wall either. For one, you really have no idea where your balls would be landing. Second, the way the ball comes off the wall in no way simulates the way a ball would be coming off an opponent's racquet. I'm not crazy about the ball machine either. Definitely work on your serve.
As for conditioning, stop eating so many potato chips! Actually, I've got a specific wind sprint drill for you. It's called the four ball drill. You place two balls on the service line, one ball halfway between the service line and net, and one ball at the net, in a straight line. You start at the baseline and either need to be wearing a watch, or have one on the ground by the baseline. You sprint up to the service line and pick up one of the two balls you placed there, then sprint back to the baseline and place it on the baseline. Then you sprint back to the service line and pick up the second ball, sprint back to the baseline and place it there. Then sprint up to pick up the third ball that's halfway between the service line and net, sprint back and place it on the baseline. Finally, sprint up to net, pick up ball, sprint back and place it on baseline. So, now you have four balls right next to each other on the baseline. Done? No!! You're only halfway there. Actually, the last ball that you brought back from the net, you just touch to the ground at the baseline and then return it to its position at the net. Then back to the baseline to get the second ball, and bring it back to its original position halfway between the service line and net. Then get the third ball and place it back on the service line. Finally, the fourth ball and place it back on the service line, and then race your ass back to the baseline. So, all four balls are back in their original position ready for you to repeat the above.
So, essentially you've just done 16 mini-sprints in this drill. A couple of technique issues. For starters, every time you pick up and place down one of the balls, you need to keep your back as straight up as possible. This serves several purposes. One, it keeps your from getting a bad back. Two, by doing all the bending from your knees it builds up your quadriceps, which are pretty important around the tennis court. You also should be facing the same alley each time you pick up a ball. In other words, if you have the balls lined up on the center T, you need to face the same side every time you pick one up. This way when you run up to pick up the balls you'll be working one quadricep, and when you place them down you'll be working the other quadricep. The other thing you need to do is make sure you actually place the balls down. If you drop them, or sloppily put them down so they end up rolling off, the drill doesn't work. It may seem kind of silly, but there's actually a degree of coordination that's involved in doing this as you're torturing yourself with these sprints.
Ok, now the really fun part. If you're athletic and reasonably quick on your feet (or at least quick enough to be a competitive tennis player), you should be able to complete this drill in 45-50 seconds. Ok, if you started when the second hand passed 12, you have the rest of the minute to try to catch your breath (you won't) before starting your 2nd round of four balls. In other words, you should have between 10-15 seconds of rest before you have to go again. If you're in decent shape, you should probably be able to do 2 of these without puking. If you're in really good shape, you could probably do 3 of these but you may end up puking anyway. If you're in phenomenal shape, you may be able to do 4 of these, but you'd probably puke.
I'd recommend starting at 2, and if you can handle that bump it up to 3 after a few days. It's actually a very good drill. Needless to say, it builds up your wind, and of course, tennis is a series of mini sprints. It's also great for your legs, BUT you have to make sure you're doing it properly. Sacrifice speed for technique, even if it seems like it gives you a few less seconds of rest between rounds (although better technique will probably make you go faster and tire you out less).