crazy fast?

So I looked up my route from Thursday, when I did the 58:31 10K. I didn't believe my Garmin because that seemed too fast for my average HR of 148. However, when I retraced the route on the USATF Map It website, it came out exactly the same as my Garmin. Wow...I guess I really DID do a 58:31 10K? Crazy fast! And it was on gently rolling hills, too. I guess I haven't lost too much fitness, which is good.