I used MP3s of CDs I had ripped to make mxtapes for my car, as it only had a tape player.
Then my car's tape player stopped working. Trying to clean the thing didn't help.
I started dating the lady that became my wife, and she had a car with a tape player in it.
However that car's tape player blew before she even owned it.
I sort of think cassette tapes just kind of blow.
The whole A vs B side of the tape thing does not exist cleanly with CDs, that is correct. Of course, you never actually flip CDs, so the cleanest way to get the same thing is to just have two different CDs. If you want to keep smaller size, you may try a "Mini CD" which usually holds 24 minutes. This isn't quite the 30/45 minutes you'd get on a cassette tape, but really, if you want something longer you can just use a separate full-size CD.
The last time I made mix tapes, I was making all tracks segue. (One of the advantages of recording from a computer.) This is more difficult to do with a CD, but really, the effect just didn't work with some songs. It was more of a novelty than anything.
The great thing about using the computer to organize the data for the mix tapes/CDs is that (1) it is easy to see how long the side is getting, and (2) when it stops working you can make another easily. If you've normalized the audio in your MP3/Ogg/Flac collection, then you don't need to worry about tracks having differing loudness, too.