Take it to the police and be with friends at all times. Its your life, so the hour filing reports may be worth it. Its your friends, if they won't watch your back, maybe they're aren't your friends.
I've received death threats before. First you want to determine if its serious or not. If not, let things be, don't antogonize the person by asking for public apologies or anything. If its serious, involve the police as soon as possible. Also, find a nice group of friends to be around you at all times...or pay for body guards if its that bad. The police can't really do very much until the assailant acts, so your police report will just help them identify suspect #1 at your murder investigation. Same with restraining orders.
Get to the gym and practice running fast on the treadmill.