1) As far as I can tell, as long as you held the pace steady and allowed for a 15-20 min warmup, you did the test right.
2) Yes, a retest at a slower pace is the way to go. That’s quite a big drift, so you might even want to try 2mph. I know it seems really slow, but the good news is that structured, progressive aerobic training as described on this site and in the books leads to rapid gains for the aerobically challenged. One note about the warmup: I also am 52 years young and I find I need a good 20 mins for the system to come online. You could try doing a 20 min warmup and then start the recording.
3) From this little bit of information it’s very hard to give any kind of answer to that question. Let’s see what your retry on the drift test shows.
One other note: are you using a chest strap and monitor to measure your HR or are you using the HR monitor on your watch? We do not recommend using the watch because it’s too far from the heart and notoriously inaccurate.