First off i want to say that i am sooooo impressed by you and what you are doig with your life at 60,,,,,sometimes people leave me speechless and mountain biking at your age? Yep you are at the top of your list. Just a thought you may want to talk to a heart doc that is referred by your regular doc and ask about their putting a holter monitor on you for a couple of days so it catches everything that your heart is doing 24/7. In my humble opinion if you were in the throws of biking when it happened it may not be anything to be alarmed about. You were doing something physical that increased the adrenaline in your body and your heart acted accordingly and within 30 min. it dropped back down...when we exercise obviously our heart rates soar and atypically it takes inside of 30 min. to return to normal limits but like itdood and Maru said the main thing is to make an appt. to be sure you are staying within those normal limits. And remember.....it is super important to stay hydrated before, during and after exercising because dehydration is notorious for causing a rapid heart rate...no energy drinks and try to stay away from fluids with sodium and alot of electrolytes in them...make the call just to get a little more reassurance and keep up the great work...i am so jealous but it is on my bucket list.....good luck
Impossible to tell without catching it on an EKG. What you describe can fit a few different types of arrhythmias.
Talk to your doctor, get a cardiology workup.
I'd recommend that you see a cardiologist.
It sounds like atrial fibrillation, might be specifically exercise induced as you are exercising when it happens.
I had this but not when I was exercising. Just had an ablation after 10 years of it.
Hopefully, you will just have it once a year!