Would είμαι περπατάω γρήγορα also be correct?
Nope! There is only one present tense in Greek, so there is no need to add the verb to be - είμαι to express any tense properties. Περπατώ (or περπατάω) will suffice.
Thanks D! So, περπατώ γρήγορα would presumably completely suffice as well?