- 今、駅に向かって歩いているところです。
I am walking toward the station. (i.e. I am on the way.)
- 今、駅に向かって歩くところです。
I am just about to walk toward the station. (i.e., I have not departed yet.)
- 今、駅に向かって歩いたところです。
I have just walked toward the station. (i.e., I have finished walking.)
These three sentences are all at least grammatical, but Sentence 1 is the most natural. Sentence 2 is not entirely wrong, but people usually say something like 今から出発します if they have not departed yet. Sentence 3 doesn't make sense at all in this context.
The existence of 今 is not really important because these sentences describe his current situation even without saying 今 (Note all the sentences end with です, which is the present tense). Even in sentence 3, it describes his current situation. It's similar to ている describing the continuation of state, except that the original action happened shortly before now (hence "just").
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