Z2 training for 50k with lots of elevation?

I've recently started doing Z2 training, and while the top of my Z2 is increasing, it's still not high enough to run big hills. Normally when I'm training for a 50k with 6,700ft of elevation gain I'd run lots and lots of uphill in my training. But now that I have to walk hills, for the most part, it seems like I'd just be hiking if I did that. Thanks for any advice or suggestions.