I know it's a pain, but keeping track of party-miles and party-hours is one way we can at least partially control for one of the factors that makes CBC data so unreliable – observer effort. If you have a watch, a GPS, and an odometer on your car, you can come up with pretty exact numbers. If all you have is a watch, you can at least estimate the miles.
For miles and hours driven by car, set your odometer to zero when you enter your area (not when you leave home). Anywhere you drive within your area counts, as you are theoretically able to spot birds and count them as you drive between birding spots. Keep a note of the time you enter your area of the CBC circle. Then write down the time every time you hop into your car and the moment you hop out of your car. At the end of the day, you can add up these times to get your party-hours by car. The odometer reading when you leave your area is the party-miles by car. Let's say you entered your area of the circle at 7:30 a.m. and departed for the countdown at 5:00. That's a total of 9.5 hours. If you add up the times you spent in the car and it comes to 1.5 hours total, then you have 1.5 hours by car.
Hours and miles on foot are a bit more difficult, because this is when groups split up into multiple parties. If you never split up, then using the example above, you logged 8 party-hours on foot. But if you split up into different groups (parties), then you have more. If you were apart for just an hour of the day, you'd have 9 party-hours. It's that simple. And a party is one to several people birding together within earshot.
As another example, say your group is 4 people. You split up, with one person hiking up one canyon, another person hiking up another, and two people hiking down yet a third. You all meet back up 2 hours later. Using the example above, the base is 8 party-hours, You have to add two more hours for each of the extra parties, so that makes a total of 12 party-hours by foot for the day.