Just some some corrections here:
I am on a walk.
我现在在散步。（It is a status that you're in the process of walking now.）
I am now walking (Change of state, I used to be sitting, now I am walking)
我现在在散步了。 （or 我现在走了=I am leaving now）。
Let's go on a walk together.
I will go on a walk tomorrow.
Yesterday, I went on a walk.
Today, I already went on a walk.
Today, I already went on two walks.
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