obelickz,
r u sure it is 6.5mph (which is 10.5kmph) or 5mph (8kmph)?
if yes, then ur leg spread is damn cool (for 162cm ppl like u).

if u insist on HIIT for cardio-based training, yet unable to HIIT run, do at 6.5mph with variety of incline. (ie. 3%-1%-5%-1%-7%-1%-9%-1%-repeat)
unable do that? HIIT with elliptical.
unable do that? HIIT with bike.
unable do that? HIIT with rowing.
unable do all above? then go fullbody with HIIT as others said (im not familiar on this).