Hi John,
I come from a similar background to you and this is also to be my first 100k. I'm of the opinion that any cross training is beneficial as we'll be clocking quite some mileage as the weeks go by, so this helps to prevent injury.
In IM training its typically 3 run, 3 bike and 3 swim sessions per week (total 9 sessions p/wk) so with the increased in running load for this race, I worked out that if i'm currently doing 5 run sessions then i still can fit in 4 sessions of either swim or bike if I want. In my case, I've substituted the bike sessions with strength sessions at the gym (was crap on the bike anyway ;P) and still keeping to the 2 swim sessions per week (total still 9 sessions).
Fri and Mon are my rest days. The rest days can be scheduled for either the swim or bike session if you like, or just rest depending on what your body tells you. Sometimes I also plan it such that I will run to the gym (whilst my husband drives there) followed by 50 minutes or so of strength session - killing 2 birds with one stone, but then my husband drives me home afterwards
For me, I feel that its important that the run sessions are to be completed as a matter of priority, followed by the strength sessions..... but the others I can "sacrifice" if I feel I need the extra rest.
