I think it would be more natural to say 「我也差不多應該走了」. The original sentence sounds like "Very quickly, I should go." Hope this was helpful!

我很快要走了

I should go soon.= 「我差不多要走了」或「我該走了」或「我要走了」(還沒走，但快要走了)，實際上要視前後文而定，就如同我們學習你們的語言也是一樣，所以，也有可能是「我該出發了」或「我該動身了」或「我該過去了」....等等。 soon = 不久、即將、很快地、過一會兒、過一下子....。 when you translate this sentence into Chinese, you don't see any words like "不久、即將、很快地、過一會兒、過一下子".... but it does include the meaning"不久","即將","很快地","過一會兒",or"過一下子"....