Oooo I have suggestions! I have a 5 day split. I do a weights session each morning for about 45min before studying (I'm a grad student), and then in the afternoon when I get antsy from sitting all day (anywhere from 3-6pm) I do HIIT or running or something cardio. So bike riding or swimming or even a weekly pole dancing class or whatever you want.
For running, I'm going 3 times a week. One day will be along run (so anywhere past 30min) where pace is kinda irrelevant.
Another day I will do what is called "Fartleks." For those, you set a timer on you phone or watch and do something like 30 seconds at a full out sprint, 30 seconds jogging. You can modify the time or have three separate speeds you do, etc.
My third day, I will be doing repeat distances. So if you are on a track, you could do three mile repeats, or five 800 meters, or something like that. If you are on a trail, you could pick a distance that you normally run in 8 minutes (using trees or houses or streets as stopping and starting points) and plan to do that 3 to 4 times. You basically run the best version of that distance that you can, treating it like a race. Then you get as long as a break as you need to catch your breath. Then you do it again! Yay :D
An alternative I plan to throw in is hills. I'm in Chicago, and there really are no hills here to speak of, so I might resort to using a treadmill. Basically, you do a full sprint up a hill and then jog or walk down (if it's a big hill and you tend to brace yourself to slow down when going down, I'd just walk and save your joints the high impact). This can be repeated as many times as you want to exhaust your butt!!!
Lastly, I have a huge list of HIIT exercises. I just made it last week and am doing my first session today. I'll be picking 2 from each category and rotating through all 6 a total of 5 times in a Tabata-like fashion (40 seconds on, 20 seconds off for each exercise ==> 30 minute workout). Here they are:
CORE & UPPER: + front planks + side planks + spider-mans + mountain climbers + clapping push-ups + elbow push-ups + kayaks
FULL BODY: + kettlebell swings + kettlebell shoulder presses + Turkish get-ups + burpees
CORE & LOWER BODY: + bear walk + crab walk + frog jumps + 180 degree squatting jumps + box jumps + swapping lunge jumps + walking lunges (and variations) + speed skater/curtsy lunge + wide squats with calf raises + stair master machine + jumping jacks + jump rope + butt kick jumps + high knees + ballet leg swings + doggie butt press-backs + doggie hydrant lifts
Ask me if anything needs clarification, or just look it up. I think the names I use are pretty common for things, but I'm not sure for all of them. Hope you find some portion of this huuuuge explanation helpful :D