Newbie HR training question

I've been lurking here for several weeks, and everything I've been reading has been tremendously helpful (although damaging to my bank account... yikes!) in getting me on my way training for endurance for the first time.  There is an incredible amout of information here.

I have been using my new Garmin 305 with the HRM for the last couple weeks with my training.  Currently I'm shooting for doing my first Marathon July 18th.  I'm following a basic training plan for a recreational runner and so far everything is going well.  I spent some time today reading up on heart rate training and have come to the conclusion that my workouts have been WAY to intense.  I've always been of the mentality that the faster I go, the quicker the workout will be over so I suppose I've really been going out too fast.  Today was my first day training based on target heart rate.  Aside from worrying that the little old lady walking her bow legged dog was going to pass me as I ran,  everything went fine.  I was surprised at how much I needed to reduce my pace to keep my HR below the 80% threshold.

My question is a simple one.  My fancy new GPS wristwatch seems to think that my max HR should be 175 BPM for running and 185 for cycling.  Is this right?  How is max HR determined? "Zone 3" (whatever this is) defaults to between 130 and 148 BPM Is this what I should be targeting?  It feels slow.  Every time I felt I was starting to get into a nice groove of running, my HR would rise above 150 and the watch would beep at me. I have no problem dialing down the intensity of my workouts to keep healthy and keep getting better, I just want to be sure I'm targeting the right levels.