Background: I have been jogging/running for the last 6 years, on and off, about 1 - 10 times a month. My typical speed allows me 10-minute miles, sometimes only 11-minute miles; my BMI is 30-ε, age 56.
I measured my max heart rate (by running 3 miles and then speeding up until I drop dead), it is 186. My resting heart rate is 45 (it goes regularly to 35 when sleeping, but I guess that doesn't count).
When I run, my heart rate is normally just above 160, and I target to keep it above 155 and below 172. I use two different devices to measure it (Scoche Rythm+ and Fitbit Inspire HR), and they normally match well (I often ran with both, to compare them).
Recently, from one day to the other, this pattern changed significantly - I run the same speed, the same route, the same time of day, but my heart rate sits now in the 130ies, for at least two miles (then it goes slowly up). It also feels easy, and the natural assumption would be I got fitter, but I doubt that - the jump literally happened overnight, from Saturday (in the 160ies) to Sunday (in the 130ies). Again, both devices agree, and I borrowed a third device to be sure - it is not a measuring error.
I can still get the heart rate in the 160ies right away, by sprinting the first half mile, and then it stays up, but unless I do that, I now run 10 and even 9-minute miles in the 130ies (since about six weeks).
Question: is such a jump overnight possible? If not, what else could it be?
I'd be happy to accept that I got suddenly fitter, but I find it hard to believe still.