If it's just an extra run, no, it's not necessarily mileage.
If it is a regular mileage run that you just happen to fill in on, then, yes, it is to be paid mileage. It doesn't matter if you are cover or not. If the run pays mileage, whoever runs it for the week or just a day, gets paid mileage.
We have several runs here, I said HERE, that go to the same relay point. Some are mileage some are hourly. It just depends on how "hot" the company thinks they are when setting them up and bidding them.
It also doesn't matter if it is over 500 miles. Just because it is doesn't mean it HAS to be mileage.
And, Upstate, as far as paying better? Well, that depends. All that really means is that you run balls to the walls, knocking everyone outta the way to get out and get back. Usually, no hooking or unhooking. Just pre/post (I'm laughing inside now) tripping and walk away. I got paid 1 1/2 for all delays, either before or after run. For instance: If I wasn't hooked up, that's a delay. If I had to unhook when home, that's a delay. Put trailers away, that's a delay. Go to the shop, that's a delay. I usually had to do all this. No big deal, I just did it and got paid in humongous $.