Yes, in most cases, mileage is included with your Zipcar trip. There's never a limit on how far you can drive your Zipcar, but there is a limit on how many of those miles are covered for free.
Most trips in the US come with 180 miles included per 24-hour period, with 20 additional miles for each additional hour (until 180 miles is reached). All additional mileage will be billed at $0.58/mile upon completion of your trip.
Most trips in Canada come with 200 km included per 24-hour period, with 15 additional km for each additional hour (until 200 miles is reached). All additional mileage will be billed at $0.50/mile upon completion of your trip.
Keep In Mind
Some pay-per-mile cars and membership plans do not include free mileage