I too get the rapid heart beat and it's not because I am walking too fast. The dry mouth happens sometimes to me but not as often as the rapid heart. Whenever that does happen I just pretty much freak out and go home. Which is too bad because that's my excercise that I do.
Same thing happens to me when I try to do ANY type of exercise. I get light headed, and my heart starts racing, and then that's the end for me. It immediately sends me into a panic. VERY annoying, as I'm trying to lose the weight I gained from taking SSRI's....
Life is NOT like a bowl of cherries! It's more like a jar of Jalepenos...... what you do today might burn your A$$ tomorrow!
This actually happened to me on the very last stretch of a hike the other day. I believe I was really triggered by the heavy breathing, which made me panic and become hyper aware of my body and surroundings. I also couldn't see the road, so I began to feel completely lost in the woods.
I hope you were able to recoup and that you feel better now!