Consistency in running is super important and this program has 3 days running built in on some weeks and 4 on others. I would say that you don't want to do less than 3 days per week running. On the 4 days running weeks I would switch out the short easy run on Friday for another cross training day. You might even want to switch the longer run to Friday and do your extra cross training day on Saturday. The big thing would be to not have a long or hard bike workout the day before your long run. Swimming would be fine though.

A program like that would get you to your goal and you wouldn't have to extend the length of the program. Especially if you're getting in a ride every week that is a little longer than the total time that you would expect the half marathon to take you.

Hope that helps.