Dude,
You've got it. Let's say you just recorded 4 bars of a rhythm on part 10.
1) hit Edit(F6), select Copy,
2) Src Ch/Part will be 10(should default to the same track you are on)
3) Src Measure will be 0001(the first measure you want to copy)
4) For is the length of measures you want to copy (4)
5) Dst Ch/Part will be the same part 10
6) Dst Measure will be the next measure (5)
7) Copy Mode should be REPLACE(since we don't want to MIX it with anything else that might be there)
8) Copy Times would be like 25 (I'd use something like 3 to get you through measure 16 = Original[4 bars] + Copy Times[4 bars + 4 bars +4 bars]).
When you said Rhythm Pattern it sounded like you meant the rhythm generator(step sequencer, etc.) not recording in Song Mode. At any rate, it should be pretty straightforward, to stop accidently including blank measures don't use ALL and only count the measures that you actually want copied.
It's also a good idea to keep some variety in your rhythm unless you are just wanting to practice over a loop. 25 bars of the same thing is going to get boring really fast.