1. Stay on the bike and run away.

2. If number one fails, get free of the bike. Use it as a fence if you can. If you can't, get away from it.

3. Weapons are your friends. Firearms are the most effective against deadly force. Pepper spray is useful as well. Knives are fine for people who know what they're for and what they're not. A knife is NOT a tool to be pulled out so you can engage in a duel with a thug who pulls a knife on you, for instance. But for a person stuck in an untenable position with someone larger, stronger, and maybe more skilled who clearly has bad intentions, a knife can be a great leveler as long as you have practiced enough to be able to deploy it when you're frightened, heart pumping, and twisted into a pretzel--or getting pounded.

4. Which martial art you study matters about a thousandth as much as HOW you learn it and how you practice it. Not to pick on Aikido, but most people who teach it couldn't fight their way out of a wet paper bag. They train to "pull off moves" against compliant opponents. A real fight will begin before you're ready and it will be chaotic. You will NOT be in control in a real fight. If you want to be prepared for that, you have to practice that way. Boxing, wrestling, judo, jiu-jitsu, kickboxing and muay thai all have their limitations, but the advantage they offer is that the majority of people teaching those arts will put you in situations where you apply the ideas and techniques they teach you against someone who is genuinely trying to win just as hard as you are. That changes everything.

5. My recommendation would be Judo, even though I don't practice it. Judo is cheap, it's nearly ubiquitous, it will teach you throws (good for escape) and some groundfighting depending on where you practice it (good for escape when the worst has already happened and you're on the bottom.) It's also very rare to find a judo school where you don't work against people who are of similar skill and doing their very best to throw you before you can throw them. It won't teach you to take a punch, but a fit person with a year or two of judo under their belt is formidable.