Basically works like this: make your guy, do some tutorials / amateur fights, Pick your 1st (choice of 2) league, then when you get the belt there you move up to the 2nd group of leagues (again you pick 1 and there are 2), and again after you get that belt you move to the 3rd and I'm assuming final set of leagues (Strikeforce and another one called Mystic), and again you only pick one of these. When you win the belt here, you stay there and defend it, eventually it is rumored that the champions of both leagues will collide in a title unification bout, but I haven't gotten up to that fight yet. Your opponents (at least in my experience so far) are mostly computer generated. I don't know if they use any of your own created fighters (I don't have any besides my career guy). I'm not sure if the timing in which you face real-life fighters is preset or not, however the 1st one I fought wasn't until the title fight of the 2nd league (aka you beat him and you move up to the big leagues), and after that I didn't face another real guy until my 1st title defense.
Generally speaking, the real-life fighters are better than the computer generate ones.
Before each fight, you do 8 weeks of training. There are a bunch of different types of training to do, and then up to 4 different variations for each training. Each of them are different, so there is a lot of variety. They all raise various stats to varying degrees. Also, each individual type of training gets a letter grade based on your performance (insert GSP joke here) ranging from A to F, which I assume changes the amount of points gained. Also, you won't unlock the next tier in that training regimen unless you get an A.
I kind of ran on for a bit there, answered a lot more than you asked
Hope I was of some help.