@Trip:
The other thing to remember is that whatever happens, you will have to use the same route again, and your assailant might remember that.
It looks to me that the combination of a sham wallet with a 20 and some cheap cards in it (no personal ID), a mobile phone you can afford to lose etc. (- the more serious the weapon he pulls, the more of your bag and pockets he can force you to reveal.) PLUS the advice to get running fit will work best.
I also like the idea about the mace, that will give you a big advantage combined with running.
Start by running in the daytime through that same area for mile or two, getting to know all the dead ends and the ways out and where they lead to.
Lol OJ Simpson used to do that as a kid and look how far he got when he was on the run!!
If an incident takes place, start to run until you know for sure the guy is not bluffing you and will pusue you. If he corners you chuck your wallet (even drop your bag) at him and try rolling out of the situation and run like fuck!! Don't laugh, rolling is taught in all armies as a key way to get out of a corner. Practice at home, place a large obstacle in the middle of the floor and roll to either side of it until you can find the angle which suits you. You can come back later with the police to search the area for the bag.
The point is the thief is out to mug you for a few bucks, not necessarily to kill you or take your books and spare clothes!!
One final point: the other posters above have indicated a fighting style might be worth learning. It is very dangerous to take on more than one attacker with a martial arts tactic especially if they possess weapons. Why? Because it's a documented fact that martial artists trying their skills in a gang mugging tend to get killed, primarily because the gang realises they have a fight on their hands, so a simple beating turns into a vicious fight in which the victim gets hit on the head or stabbed etc. People like that often hate their victims enough to not expect them to fight back. They have nothing to lose, you do.
Besides you don't sound like the type who has the body or the personality to for street fighting!!