How to extend a logical volume?

I recently upgraded my Zimbra email server with bigger harddrives.  The configuration was 2 250GB drives setup as a mirror.   Using clonezilla, I cloned a bare metal image of the server and then replaced the drives with 2 750GB drives.  I then restored the cloned image back to the 750z and proceeded with testing the system and so forth.  Everything checks out except when using Gparted you can see the unallocated space left over from the bigger drives which needs to be merged into the volume.  Using Gparted I tried to resize the volume to use the remaing 500GB of unallocated space but what happend is now I have an extended partition of 750GB.  I need to have my logical volume utlitze this space.  How do I properly go about accomplishing this.  I have read about the lvmextend command and how to extend the lvm but I don't know how to properly issue the right commands to makes this happen.  Any help is appreciated.

[root@mail ~]# parted -l

Model: Dell VIRTUAL DISK (scsi)
Disk /dev/sda: 750GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End         Size         Type     File system  Flags
 1      32.3kB    57.6MB   57.5MB   primary   fat16            boot
 2      57.6MB   2213MB  2155MB  primary   fat32            lba
 3      2213MB  2320MB  107MB    primary   ext3
 4      2320MB  750GB     748GB    extended
 5      2320MB  249GB     247GB    logical                         lvm
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