You can also say 'It takes 30 minutes on foot.'

If you are talking about travel distance most people say, "by foot". For example, to give directions you would say, "To walk to the library, it takes 30 minutes by foot". If you are talking about standing time most people say, "on my feet". For example, to describe your day at work, you would say, "I have been standing on my feet for all day!" Hope this helps

It takes 30 minutes to go there (additional) on foot.