The acute effect of exercising (any type of exercise) is that the muscles become engorced with blood. This serves to increase oxygenation, remove metabolites, deliver nutrients and anabolic hormones. A muscle that is engorged is, obviously, larger and tighter than one that is not. This is the reason behind your "toned" muscles".
Another thing that happens during muscle contractions is that the neural imput it recieves increases. This has the effect of an increased post-exercise tonus; tonus is when a proportion of the muscle is contraccting, even in the absense of force production. This is another reason why your muscles feel harder after exercising, but this effect also disappears after a couple of days (even faster with warm showers and massages).
Basically, there is no change in the structure of your muscles the way you train. You exercise, and then wait for the effects to completely disappear before training again. In order to progress, you must stress your body. I would suggest you read up on "supercompensation". This is called the one-factor model of training and is kind of outdated, but still very similar to the more accepted two-factor model of training. However, unless you plan on becoming a professional, there is a minimal difference as far as results go.
On another note, I see that there are a couple comments about toning for the previous answer to this question.
"Toning is a myth" - Yes, in the sense that people portray it. Toning will not increase your fitness markedly, nor will it get your muscles stronger or harder. What people refer to when talking about toning is when slow twitch muscle fibers undergo sarcoplasmic hypertrophy. So, in one way, performing low weight/high rep exercise can increase muscle cross section area. It will not however increase strength. Also, the hypertrophic potential of slow twitch muscle fibers is much lower than that of fast twitch fibers. Aiming to get "toned", in the sense that people do, is a waste of time. Classical body building methods, medium weight/medium reps, are more efficient.