I would suggest that they have the same meaning but slightly different grammar.
the first - I'm talking a walk on the beach
In traditional grammar, there's a subject, verb, object and prepositional phrase. The verb would be analysed as present continuous, suggesting an action that is taking place at a point in a time, a description of what's happening now
In systemic functional grammar, this clause would be analysed as:
I - actor
am taking - material process (=action verb): present in present (same as above)
a walk - range (this term is used to identify the element that specifies the range,scope or domain of the process. It suggests that this element is not "so much an entity participating in the process as a refinement of the process itself. This may be the name of a particular variety of the process, which being a noun can then be modified for quantity and quality eg. have another long walk" Halliday & Matthiessen, Intro to functional grammar 3rd edition, pg 295)
on the beach - circumstance of place
the second - I'm walking on the beach
In traditional grammar, there's a subject, verb and prepositional phrase. The verb would be analysed as present continuous, suggesting an action that is taking place at a point in a time, a description of what's happening now
In systemic functional grammar, this clause would be analysed as:
I - actor
am taking - material process (=action verb): present in present (same as above)
on the beach - circumstance of place
As mentioned above, the use of the structure take a walk, allows the noun walk to be modified for quantity e.g. I'm taking another/my first walk on the beach or quality I'm taking a long/tiring/exhilarating walk