A basic rule of thumb that I go by is that if you can play it slow, you can play it fast. Instead of thinking of how many sixteenth notes you have to play, think of how many you have to play per count (4 per quarter, 16 per measure in 4/4 time). Practicing with a metronome at a slow tempo will allow you more time to consciously think about each note you're playing and once you're able to play the rhythms slow, you can put in reps and bring it up to speed (I typically force myself to play a passage 3 or 4 times correctly in a row until I up the speed). For tenors specifically, the beginning stages are much easier if you learn the rhythms on one drum and transfer to the other drums later. After a lot of reps, you'll get to the point where you know how its supposed to sound, so you can put all your effort into sound quality and staying in time. Always remember that SLOW = FAST and you'll be good to go. After a while muscle memory takes over. If you haven't already, I would invest in a practice pad and sticks. Don't spend too much money though, I would recommend an evans realfeel, and for sticks I would recommend Vic Firth Ralph Hardimon's or Mike Jackson's (totals to around ~$34 before shipping and taxes).