I'm guessing (but maybe you can confirm) that having trained for a marathon, (a) your running fitness may be better than your cycling fitness, and (b) you will have a good endurance base. Take this as a cue to your training over the next 2/3 weeks - I would prioritise cycling over running, perhaps do a couple of long rides in place of long runs. For both cycling and running try to do some tempo/threshold level efforts as this is about the level you'll be working at for the race.
W.r.t. practising running off the bike, it's good to experience it so that you know what it feels like, but IMO it's more important to be cycling fit + running fit, than to worry too much about going from one to the other. You're either gonna be jelly legged or you're not! On race day, it's a good idea to err on the side of high cadence on the bike, rather than pushing a big gear, and spin the legs out before starting the run, so that your quads aren't too heavy.
And finally - if you're changing footwear in transition - elastic laces!
