The condition you describe may be medically called an arrhythmia (fast and irregular heartbeat). The danger of a very fast heartbeat is the risk of the heart's inability to have enough time to adequately fill with oxygenated blood and as a result there can/will be reduced cardiac output that would cause a serious medical condition and symptoms of shortness of breath, Chest pain, Dizziness, Lightheadedness, Fainting or near fainting, Blackouts, cardiac arrest, etc.
The fact your tachycardia (fast heartbeat) is the result of exercise and not very, very fast may bode well for not being a serious health problem. You should see a cardiologist before resuming any strenuous activity. If one is not in good physical condition and that could be an underlying cause.
Thanks for sharing. Hope this helps and take care.