i am new to running but a regular cyclist mostly 10 and 25 mile tt's.
in most open events on sporting (i.e. not dual carriageway) courses but more around the lanes or closed circuit with drags and bends, to get in the top 50% a 10 time of under 25 minutes and a 25 time of under 1:03mins is about the target.
we all try to get under the hour for a 25 but it's tough for "average riders" especially i suppose if you have just ran 3 miles.
to reverse your question, what time would the ave runner do his/her 5k in under duathlon conditions, i tried it once and managed 21:50.