Hello BluesFan,
under certain circumstances, such as being on 'retreat', you can meditate more than the usual 20 minutes twice per day. However, there are all the self-pacing issues that go with that sort of thing as discussed in the lessons.
What I am saying is that 'going on retreat' like that should not be done lightly, or without some prior experience of meditation first, or guidance.
Even if 'on retreat', most people find that optimal results are not obtained by really long meditation sessions, but rather by a larger number of shorter ones. They should be separated by asanas and/or walking, or mindless working (sweeping and cleaning can work while one is 'on retreat', but obviously, many kinds of regular work are not suitable -- even cooking is not usually suitable, unless it is simple and easy the thinking and sorting have been done in advance).
In zen, they have gotten this down -- they never meditate for a really long time, and always break it up with walking sessions. If you are lucky enough to have a zen monastry nearby, you might be able to go to their retreats and do mantra yoga quietly while you 'sit'. I have often done this. Most of them are tolerant of it, though they don't have to know if they are not.
Regards,
-David