Part of it will be what works for your body, and what keeps you motivated. And keep in mind that tris tend to be my focus now, and any running races I do would be in a mix of tris. But I'd say try incorporating a half marathon plan into your current program. I find that swimming and biking helps me get a bit of extra fitness, not get bored, and prevent some injuries :)

I faced the same problem at the beginning of the year. I was training for for a mini marathon and also trying to try to train for my first tri. Since my main focus at that time was beating my half marathon time... i focused on my run plan. On my Cross Training days i would throw in swimming or biking. I was confident that i could get through the bike with no problem but i didn't know how to swim so i tried to get more time in the pool if i had to choose. I say just stick with a running plan and on cross training days thrown in a bike or swim! Good Luck!