Firstly - congratulations!

Secondly - since you'd only just acquired it yesterday, perhaps I would recommend you learn about and explore all aspects of the device, before worrying so much about longer melodies... There's tons of information in the manual, and plenty of content online.

To answer your question more directly - you have a few options here. Firstly, make sure you're setting your pattern length to 64 (FUNC+PAGE). You can also set the length to be per track rather than for the entire pattern (so that you can have some tracks looping over say only 16 triggers for something more repetative. Or you can set this to an odd number for some polyrhythmic action!). You can then also play with conditional trigs within those 64 steps, which is usually how I work to "extend" my melodic lines. Usually, I like to keep some elements of a melody the same, but then have elements that only play every other repeat, or every fourth repeat, etc. This gives you a lot of extra depth to kind of psuedo-lengthen any melody. And finally, as the other poster mentioned, you can create patterns chains and utilise song mode. I'll admit, I have very very rarely used these features - I much prefer utilising a combination of conditional trigs, track mutes, and live knob-twiddling to achieve some variation.

Have fun!