Use Encore's "Automatic" setting.
2 hours is pushing it for a single layer disc in terms of quality, and will result in a bitrate of 4.82 average with VBR encoding.
(Encore will automatically use VBR encoding at anything under 6Mb/sec as a rough guide)
Whether or not this is acceptable will depend on your source footage.
If there is a lot of rapid motion & fast stuff, or heavily detailed scenes, it might not be sufficient.
To test, try taking the busiest clip you can & transcode it using the following settings:
Peak Bitrate = 9.41Mb/sec
Average (Target) bitrate = 4.82Mb/sec
Write to an RW disc, and see what it looks like.....