There is no answer to the question of what is “fast”. A world class male marathoner will have an aerobic threshold pace of about 4:50/mile. Thats right: 4 min and 50 seconds per mile and that is a apace HE can maintain for 2+ hours. I think we can all agree that that qualifies as “fast”. We frequently see folks who have very little aerobic training history start with us in the 12 min/mile pace range at AeT and after a few months see that drop to 10min/mile. But, like everything else when it comes to one’s physiology: It is highly individual. You will not learning anything about yourself (or your fitness) by comparing yourself to someone else. You need to compare YOU to YOU.
The best method is to go to a lab and do an aerobic threshold test or a metabolic efficiency test using a gas exchange method. The next best is to do a incremental step test in which you measure your blood lactate at ever increasing speeds/HRs. There are articles covering these tests in detail on this website. These will tell you about your own unique metabolic response to exercise.
The HR drift test is simple and does not require a GPS. It only requires a treadmill and a HR monitor. Start with a nice gradual warm up wherein you settle on the pace/HR that you feel should pretty close to your AeT. Then hold that pace for 60 min and record your HR. Using Training Peaks you can see what your average HR was for the first 30 min and compare that your average HR for the second 30 minutes. If the second 30min average is more than 5% greater than the first 30 min average, you were above your AeT for this run. You may need to do this test a few times to dial it in. We use this test with many of our coached clients and it shows their AeT improving over time and is nice simple test as well as motivating.
So, don’t worry about what someone else’s “fast” is. Do one of these tests to set a base line and then re-test in a few weeks/months. The HR drift test is simple and cheap.
I hope this helps.