running with a HR montitor for the first time - lesson learned!

Today was my first time running with a HR monitor (I tried my new Garmin 405 Forerunner) and what a difference it made! I started the run as usually, warm up at 4.5 miles per hour for 5 min and then run at 6.5 miles per hour. After about 5-6 min of running at that speed, I was at 90% of my max HR! No wonder I've been having such hard time running continuously for longer than 10-15 min, I was going too fast and my heart was working too hard! So I went down to 6.0 mile per hour and was able to rung without a walking break for 25 min or until I completed my goal with was a 5K run (or just over 3.1 miles). Actually I could continue running for another 25 or 30 minute as I was feeling so great and my run was so effortless but a throbbing pain in my right Achilles tendon (on the back of the ankle)stopped me. I think my running shoes are too high in the back and are pushing against my ankle causing my Achilles to get irritated and inflamed. I had this in the summer when I was running outside but this time is very bad. Actually, I am in a lot of pain now and moving my ankle makes it worse... so walking and going down the stairs is very difficult ( and we live in a townhouse with 4 levels so imagine all the stairs at home).
I talk about my runs in terms of kilometers as I am more comfortable with it but my treadmill is in miles so I am reporting speed in terms of miles, which can get confusing :)

So what I learned today is that I need to slow down and slowly build up my speed. I don't know why I never got a HR monitor (and I am a "gadget" person) but now that I have it I will use it all the time. I just hope my ankle injury is going to go away soon so that I don't have to wait too long until my next run :)