Regular exercise is better than (occasional) bouts of strenuous exercise.
bout - a short period of intense activity of a specified kind Source of meaning. Also, the exact sentence is present here.
Regular exercise is better than a burst of strenuous exercise.
a burst of - a sudden increase in something, especially for a short period Source of meaning (credits to @JavaLatte, because he said it first)
Regular exercise is better than a flurry of strenuous exercise.
a flurry of - a sudden, short period of activity, excitement, or interest Source of meaning
Regular exercise is better than rounds of strenuous exercise. (not very relevant, but can be used)
round/rounds - a number of things or group of events (done successively) Source