Not bad indeed as first song, and it has quite a good structure so far compared to my first song (where the entire song would change about three times, when in your case, the whole idea remains intact until the end). By the way, my first 3 songs used mostly pre-made riffs and even now I sometimes take pre-made riffs as reference then modify them, although I have pretty much completely eliminated any pre-made riffs from my melodies since late 2003. The last two notable cases where I did use modified pre-made melody/bass riffs were the blip sound in the background of The Summer Trance and the acid sound in Epic Journey Through the Galaxy somewhere after the first parts. New Beginning 2012 also uses a pre-made riff, but that's kinda inevitable considering it's a remix of my 2002 song New Beginning.
And yeah the riff editor can be annoying at first. First of all, you must backup often (or at least save your progress every 20 minute or so) because this software freezes often when running low on memory and it's better with the instruction manual. It pays off after getting used to it, though, since when using pre-made riffs, the memory gets cluttered fast.
Good luck in your MTVMG projects!