歩いて二十分ほどのところにある川原です。
It's a riverbank about twenty minutes away by foot.
I have a couple of questions with the usage of 歩いて here:
I believe that one misconception on your part is that you seem to think that there are two attributive clauses (歩いて二十分ほどの) and (歩いて二十分ほどのところにある), while there is in fact only one: the latter.
One easy thing to do in order to understand where subordinate clauses start and end is to remove the parts you think constitute subordinate clauses. Then check if these parts form grammatically correct sentence on their own, and whether or not the rest of the original sentence remains grammatically correct even with these parts taken out.
The part 歩いて二十分ほどの can't form a grammatically correct sentence on its own. With that taken out, the remaining part of the original sentence will be: ところにある河原です, which doesn't make any sense either. If we however try to split the sentence at 歩いて二十分ほどのところにある, we'll be left with two parts which constitute grammatically correct sentences on their own.
To answer your question: There is no omitted verb in the original sentence.
As for your second question, 歩けば二十分ほどのところにある川原です is grammatically correct. There is a slight difference in nuance between this sentence and the original one; This sentence emphasizes the fact that 20 minutes is how long it will take to travel on foot (in contrast to other means of transportation).The original sentence contains no such indication, but is simply a statement of how long it takes.
歩いて here simply means "by walking" just as バスで would mean "by bus".
