<human>: Provide a brief overview of the text.

I'm 50, and I just struggled through week 4 of C25K. trouble is, my heart rate monitor says my ticker is gonna explode. I hit 150 bpm almost as soon as I start running, and it's between 170 and 180 bpm in the last 5 minute run segment. And I'm running *very* slowly.

I'm not overweight. I'm 5'10" and 160 lbs. I lift weights, nothing impressive, but the point is I'm not exactly a couch potato. Blood pressure's good, cholesterol's good, iron level are sometimes low so I take supplements.   

I'm concerned with the high heart rate because (a) it seems a little dangerous, (b) I might just be training my anaerobic capacity and not building basic endurance.  

If I walk at 4 mph my average heart rate is 128 bpm. Walking feels easy - too easy - I can keep going for more than an hour and it doesn't seem like I'll make any progress just walking. (Also it's boring.)

Maybe my max heart rate is just higher than normal. Does that happen? Should I do some sprints, find out what my *real* max is, and based my training zones off that? Does anyone actually test their max rate or does everyone use formulas like 220-age, or 180-age?
<bot>: my heart rate zooms up whenever I run. Walking is OK, but it feels too easy and I think it won't help my endurance.