We reached our goal after two days of hard walking.这里为何不用名词walk,而用walking动名词?

I think:
We reached our goal after two days of hard walking.
We reached our goal after two days' hard walk.
虽然walk也可以是名词,但在of后习惯用动名词形式,而第二句也是常见形式
如：
30 minutes' ride
It takes about 2 hours' walk.