Keeping fit on minimal training

Quarters at tempo pace with short rest. Do them every week or two. Low mental stress, still gets your heart rate up and let's you get some turnover in.

If you don't have access to a track, just do a fartlek of 90 seconds at tempo effort, 60 seconds jog.

Then I would have done as many of those runs as short intervals at a controlled effort. Like 10min WU, 25-30x45/15 and then 5-10min CD. Start at a comfortable threshold pace (like 2h race pace) and progress the pace as you feel for it to 10k and even 5k pace. Sometimes you can just do them easier and sometimes you can go quite hard and end up with a VO2 max workout. There is something about the mix of 45s running and just 15s rest that feels like being optimal. 45 is not too short so it can be too much of sprinting, nor it is too long to build lactate, then 15s rest is not too long to lower the HR or VO2 consumption, but long enough to give the muscles a break so they can relax and stay "soft". So it feels quite comfortable, but still one can run at a quite fast pace. You can do this on a treadmill too since 15s jump-off rest is easy. Also you could do it as alternations of 45 faster and 15s slow or you could do 30s faster and 30s slower.

I do 45/15 at threshold effort and end up between my 10k and 5k pace (which is approx my 30min race pace). You can do them flat, but also hills or downhill.

If you have very limited time you can do 5min WU, then 45/15 from a comfortable pace and progressing and then just do some minutes of CD jog. Even in 25min time you could have done a good workout. If you vary the pace a little you could fit 3 times of 4 in a week and then just do 45 easy the 4th. For ex 1 faster, 1 slower and 1 medium pace + 1 easy.

If it gets boring you can of course vary this, but in terms of keeping the ability I recommend 45/15

Is the 15 a slow jog or a walk?

The minimum health recommendation is 3 times a week 30 minutes. Below that it makes not much sense.

Assuming you run 3 times/week you can do 2 times easy runs and 1 time hard run (HR around 90-95% HRmax, never forget to warm-up and cool down)

I stand, one could walk or jog slowly. I think it is important to give the muscles a break and not run at all too fast in the rest. Jog or standing has probably no effect difference other than reduced recovery. If faster or done as alternations it changes the workout and I would probably try 40/20 down to 30/30.

Probably there is also a psychological aspect in that the mind kind of thinks this workout is easier than it is and fatigue seems to set in later. I am often suprised of the paces I can hit comfortably with this workout. So it gets like a workout giving turnover and speed for the legs without the recovery need