18 week Sweet Spot plan-adding some running?

I just started the 18 week sweet spot plan. I usually focus on triathlon but chose this plan to improve my cycling. I chose the basic plan so I would have time/fitness to run atleast twice a week. Do you have recommendations for which days would be best to add a run? Also, I anticipate some weeks I may have to substitute one bike work-out for a run due to family obligations. Which bike day is least detrimental to skip? Thanks!