Cherish it. It's not a way of measuring how effective your workout was, but it should tell you that you've done something out of your comfort zone, and that's good!
The more you train, the less of it you'll have. But generally, don't worry about it.
This is important. Learn this immediately!!
When you train, you break the muscle fibers down. When you rest and eat, you build the muscles back up, stronger.
So essentially, you failed your pushups, because your chest and tricep muscles were already weakened from your previous workout(s).
We like to go to the 48 hour "rule of thumb", and suggest that you don't train the same muscle groups two days in a row, but give them at least one day of rest before you train them again. This can be overlooked if you eat and rest a LOT between each workout, but again, the 48 hour rule of thumb is a good place to start to see how you handle it.
2 kg in 1 week is fine, but it goes under "easy come, easy go". A week is a very short period of time, and what you accomplish in a week can be lost in another week. But what you gain in a year, sticks with you for much, much longer.
Obviously, quitting smoking is the way to go. You know it, we know it, I don't think it warrants too much of a reply, besides... Good luck!