Question about going slow

The reason to run "slow" is so that you can run more.  So unless those shorter running days represent cutting back what your volume would be in your normal schedule, then the only reason not to run them at your "slow" pace is because you have a harder session planned. 

30-45min runs are great ones to run easy and add volume with relatively low cost to the rest of your weekly workouts.  And 30min is not "too short" (nor is 20 minutes for that matter).  Anything is better than nothing.  Adaptations will change as you go longer and/or harder, but postive benefits can come from any training.

Going "long and slow" is about putting your body under stress for an extended time.  This is true and doing so provides valuable adaptations.

However, you can also derive valuable adaptations when you run for shorter periods of times, just not as much as when you run longer.  And this is partly because you are producing less stress.  But by generating less stress, you also can recover faster.  So you can get some valuable training adaptations at relatively little 'cost' to the rest of your training.  If you ramp up the intensity of those shorter sessions, you increase both the stress and the recovery needed.  Balancing stress and recovery are the keys to any training program.  Generally, it is better to run 'more' than it is to run 'harder'.  So short, easy runs ("short" and "easy" are both relative to the individual) can be a very valuable part of a training program.

That said, if life forces you to cut back on your typical volume for some reason (or you have reached a level of volume that takes up all the time you have to commit to the sport) then you can make up for some of that with some higher intensity.  'Some', in this context, also can vary quite considerably given the individual, their backgroud, current volume, etc.

The only time I have anything that short on my schedule it is pretty much always a "recovery" type run which is done at a guilty slow pace.  It is basically something for me to loosen up the legs and keep the muscles firing and help them heal up faster for the next intense effort.

But if all you can fit in is a 30 - 45 minute run, then make the best of it and don't waste the time.

Honestly though, if you are trying to make it into a faster session than will not get nearly as much out of it in my worthless opinion than if you just ran a nice consistent pace.  To do any sort of faster/tempo style work you need at least 15 minutes to warm up and maybe another 10 to warm down which only leaves you 5 - 15 minutes for the faster stuff, which is really not worth the effort to me.  30 minutes at a decent aerobic effort will probably reap better long term gains than trying to cram a short speed session in (and avoid injuries much easier).