I think that the process of doing exercise takes some time until the person can be said to have become fit. For this reason, I think (b) is better than (a). We use cause-and-effect, or type 0 conditionals, for conditions leading to immediate and certain results, for example:
Thanks a lot Gustavo, but what about the following sentence that Michael Swan wrote in Practical English Usage page 256: "If you eat too much, you get fat." I think getting fat is not the immediate result of eating too much.
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