Sounds like hill repeats are going to be your best option - driving to the long hill would be great, but probably impractical.

Here's something to try to make your hill repeats more challenging/interesting/effective: time yourself the first time you do it. Ride at a comfortable pace you'd think you could sustain for your 8 mile hill. Then try to keep your subsequent repeats at the same time as the first one. It's not easy! Do as many repeats as you can the first time. The next time, add one more. And so on...