I believe you can quickly eliminate D & E because "when" should only be used for time or a specific event.

Eliminate A as those does not really refer to anything. In addition the comparison is wrong. "running...saves...compared to those who do not". Do not what? run? save?

Eliminate C for an incorrect comparison. "Those running save costs...compared to not running". Those is also ambiguous. Who or what is running.

B has the correct comparison as it compares two sets of people- "nonrunners" and "people who run"

I agree. D and E can be eliminated first since "when" is wrong.

The next thing I looked for were modifier issues since the original had "Compared to those who do not run, ...."

A - modifier is not describing "running"
C - modifer does not describe "those"

B is the only choice that has a phrase describing a noun ("people")

Correct one B.

This information may be beyond what is needed for GMAT, but one way you can look at gerunds is looking at them as noun phrases.

In answer A "running three days a week" is simply a noun phrase, thus cannot be compared to "those who do not", which presents people.

I think C can potentially be correct if "those" is deleted:

Compared to not running, those running for three days a week save