"I am wandering in the park."
The accusative case is only used for specific direct objects and some fixed time expressions. This is neither of those things. It does not translate as "the" all of the time (or rather "the" does not translate to it).
Also, nouns can only be in one case at a time.
It means you are aimlessly walking around :)
Then maybe dolanmak is better for wandering and gezmek is better for walking around. Because gezmek has the objective of seeing things and having fun in itself.
I guess it is actually a vague term in English now that I think about it. "Wander" can also mean to walk around, normally using different paths to reach a place (especially if you use "over" with it :D This sentence is probably better with "walk around" though :)