Also, go out and run a few miles as fast as you can handle. You want to figure out about where you max heart rate is. Those age based formula don't work well for everyone.
I did this the other day, and I ran about as hard as I could for about 2 miles, which included some inclines as well. The max heart rate I could get to was 182. So I'm assuming I'm subtracting my age (44) from this, which would be 138.
I'm finding it extremely difficult to hit that number. Downhills or flat ground I can, but the pacing is so dang slow. The only way I can hit it on an uphill is if I darn near walk.
Is this normal, and just part of the process? If so, really frustrating. But I'm guessing beneficial, based on the reading I've done here and across the web.