becky,
I suspect you have your max HR wrong. Beware of formulae - they are rather unreliable. Also note that Hadd talks in terms of % of max, not working heart rate. You should be running at 70% of max. 85% of max should feel you are pushing the pace, whereas 85% of working heart rate will be more like 10k speed.
It does sound like you should be running faster, ie at a higher heart rate.
Try doing a Hadd test - laps of progressively higher heart rates - the details are in the Hadd article linked from page 1 of this thread. This will give you an idea of your lactate threshold, and you can base your pace/heart rate on this.
Another option is to run a 10k race wearing a HRM. If you run a good race (ie well paced) then this can give you a fair idea of your lactate threshold - the HR you can sustain for 1 hour.
You could try a max heart rate test, but this is very hard work - you have to push so that you almost black out.