Is biking a 41 minute 20k ok?

It depends on how fast you want to ride. If you want to ride at 30kph then it's great.

Can you keep that speed up for 4 or 5 hours?

Keep training. Remember, you need endurance first, then speed.
 
That is pretty good. It's better to take it easy and then push yourself harder each week. This will bring your mentality and morality up. Keep it up man!
 
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