I would divide intervals into anaerobic and aerobic. Anaerobic being shorter and higher in intensity with more recovery between work bouts. Think sprints with walk recoveries. Aerobic intervals are longer but not as fast with jog recoveries. Hill repeats are also worth talking about if you want to develop power but the injury risk is higher. Which is kind of the rub for interval stuff. Yes, its more effective than steady state but it also requires more recovery, so there's a higher cost to it and the injury risk goes up as well. Steady state will make you a more efficient runner and allow you more training time but again, without defining goals its a moot point.

For a general physical preparedness training program I'd try to do three runs a week. One steady state, one anaerobic interval and one aerobic interval.

For steady state, I do about 30:00 jogging at about 6.3mph (9:31 p/m) on a treadmill or a stationary bike keeping my HR in the 120-130s range.

For anaerobic intervals, I start with that slow jogging pace for a 5:00 warm-up. Then a 1:00 interval at 9.4mph (6:22 p/m) increasing this with each interval and finishing the last few at 10.6mph (5:39 p/m) for 10 work intervals in total with 1:00 walk recoveries in between. Then another 5:00 cool-down at the jog pace. The big mistake people make with these is that they aren't hard or long enough to be beneficial, but there's a very fine line there with injury risk.

For aerobic intervals, I do 1/4 mile at the jog pace, with 1/2 mile at 8.6mph (6:58 p/m) x4 with a 1/4 mil jog recovery. This isn't as fast as the anaerobic intervals but the recovery between work bouts isn't as complete. These type of intervals runs replaced the tempo runs which were previously prescribed in a lot of training plans, which are designed to train your ability to clear lactic acid. Aerobic intervals accomplish the same thing but are easier to recover from.

With any program there has to be some kind of progression, be it duration or intensity. Combine this with strength training and an active recovery day here and there and you'll do fine. To define what an active recovery day is, I wouldn't do a workout but would wash the car or cut the lawn etc.