MacBook Air - Time Capsule backup takes rather long

A Thunderbolt to Ethernet adapter is what you need if you want to establish a wired connection between your Mac and one of the three LAN <--> Ethernet cables on the back of the Time Capsule.

Thunderbolt to Gigabit Ethernet Adapter - Apple

If you decide to use the adapter, be sure to turn off the WiFi on your Mac to insure that the connection will go over Ethernet. When the first backup is done, you can disconnect the Ethernet cable / adapter and turn the WiFi back on at your Mac, since subsequent, incremental backups will normally only take a few minutes, unless you have just added a lot of new data to your Mac since the last backup.

If your Mac is connected to the Time Capsule using a wired Ethernet cable connection, you can expect backups to occur at a rate of about 30-40 GB per hour, so an 89 GB backup will take about 2-3 hours.

The bets are off with wireless, since the speed of the backup will depend on the quality and speed of your wireless connection. In very general terms, a good wireless connection will usually take 2-3 times longer than a wired Ethernet connection, so 6 hours for an 89 GB backup would be right on target.