Have you had any kind of ekg, echo, or other test to see if your heart is functioning correctly? If you haven't, I would get it checked out right away and rule out any structural problems. When I've had arrythmias and fast heart rate, I knew my heart was sound because I've gone through all the tests. My problem was an electrical one. So for me, with your symptoms I would not be alarmed. But for you, if you haven't had all the tests I'd go see a doctor or even go to er if it is still bothering you as soon as possible just to rule out anything that could be dangerous.
How fast are we talking? Is the rates jumping around or are you just feeling skips?
If it's really fast and the rate itself is jumping around and it's still doing that after many hours you might want to head to the ER to make sure you aren't in AFIB. If it is it might be something called lone afib. They can give you med to convert it back to normal sinus rhythm.
If it converts back on its own, which is might, head to the docs for a checkup.
I agree you probably over did it. Maybe you just need to try walking on flat ground instead of walking up steep hills. If you havent worked out in over a year, you may just need to work on your cardio alot slower.