Firstly congratulations on the marathon and deciding to do a tri. Both are great things to train for.
For most tri's the bike to run transition is the most difficult (have you tried your first run off the bike yet?
I still remember my very first time - 18 years ago!)
For a sprint you won't need to put in any mega mileage, and with a marathon background you would get away with a few single-training-session-days each week. I would definitely recommend practising the bike to run transition frequently. Also try the swim to bike transition and see how it feels. Different people cope differently with this - for example the swim to bike never seems to bother me too much, and as it is a logistical problem for me to practice I rarely bother.
Transition practice should be used to not only get used to the feeling of running on 'cycling legs' but also to practice the technique. Lay out your gear like you would in a race and practice so you go on auto-pilot race day.
Once or twice a week for this should suffice, although this will depend a little on your goals and your strengths/weaknesses.