The most effective way to vary the length of the sentences in this passage is to combine sentences 2,3 and 4 to create a series.
Since these sentences include a series of items of the same characteristics, they can be organized in a list, arranging them in parallel form. This list would be introduced by a colon and each item would be separated by a comma or, ocasionally, with a conjuction.
The final version would be:
Excercise provides many benefits: it strengthens muscles, it helps prevent unhealthful weight gain, and it may reduce the risk of some cancers.