Translation:Marcus in platea celeriter currit.
Marcus in via celeriter currit. A couple questions: 1) Is "in via" correct? and 2) Is this dative case?
Accusative would be "ad viam" I believe.
Using in when specifying the location something is happening (no movement), the ablative is used. So here in platea and your in via are ablative. (The dative of via and platea are viae and plateae respectively.)