3.3 w/kg equates to a run speed of around 25 minutes for a 5km so it should be fairly easy for you. When I was quite bike fit but not run fit though I could easily go out and smash one run, but I wouldn't be able to walk for days afterwards. I'd definitely agree with the posters who said just go and run super easy/run walk for a few kms a few times to get used to it, and then get out and smash it on race day.

ETA And I would definitely run with a pacing tool (strava on iPhone?) and run at exactly 5min per km, no quicker, for the first 3 1/2 km and then slowly wind it up over the last 1 1/2 to come in 30 seconds under goal.