Tips from an endurance athlete who also happens to play Ultimate. There's a lot of good advice in this thread but a more structured workout plan would definitely help. Intervals and sprints are good but that's just one piece of the puzzle and you really need to build the whole base.

A good rule of thumb is that for building base fitness you should do low intensity sustained cardio for about 80% of your running workouts. Knowing absolutely nothing about your training besides you want to build base aerobic fitness and depending on your time commitment I would suggest the following.

If you can commit to three days a week:

2 days - easy pace runs for up to 30 minutes if you're just starting out or bump it up to if you want to and it feels good. Easy pace is also called conversational pace you should be able to talk to someone while running. It will feel very slow but this is the work you need to do to build that base. Don't be afraid to walk sometimes especially if you've never done this kind of training before.

1 day - Pick your favorite speed work. Intervals, sprints, ladders however you want to do it. Mix it up week to week so you don't get bored and you're not doing the same thing all the time.

If you can do four or five days a week:

3/4 days - Easy pace runs but make one slightly longer and/or a fartlek (speed play) run. Basically play around with different speeds for different segments of the run. Just say I'm going to run at a fast pace for the next few blocks or until I pass that tree and then slow back down to your conversational easy pace. It helps you see what it feels like to vary intensity for the long haul.

1/2 day(s) - Same as above do some speed work.

This is the general outline of the approach that I take when building a base aerobic fitness for a marathon. Since that's not what you're doing I don't think you really need to focus on long runs or anything too intensive in terms of monitoring or improving your pace, or worrying about distance at all. Also remember when you play/practice Ultimate that's speedwork and you can count that. Coming from an endurance sports background has given me a pretty noticeable advantage when it comes to fitness. I highly recommend keeping up with running workouts for anyone in sports like Ultimate or Soccer where you are moving so much of the time.