Newbie with a question about setting intervals

Hi guys, So I'm sort of new to this whole 'super intense swimming' thing. I swim on and off over the school year, but this summer I've really been trying to up my game and I've been going to the pool everyday for about an hour. So far it's been going great, but I'm having some trouble picking appropriate intervals for myself. The focus has been on working on my endurance thus far, however swimming straight miles got really old fast.I could do a mile in a little under 45 minutes when I started. I then changed it to 15X100 on 2:50 which was a good pace, but not terribly challenging. I did a sprint workout today, which was nice but only the rests were timed. I timed one of my 100s once, and I swam in 2:14 which I know is pretty awful. Basically, I'm hoping to speed up my longer distance stuff, if only for the sake of being able to cover more distance in each workout and do more stuff without being in the pool for 5 hours. So if anybody has any advice for me about this or just in general, I would love to hear it. Thanks! Science Geek