Hello damnitchloe - sorry to hear you're feeling down & exhausted......It sounds like you're making every effort to do all the right things (eating good stuff, resting, drinking water, exercising appropriately) so it must be super-frustrating to still feel so drained.
The only thing I can suggest is that you might be anaemic? If you're 20 weeks PG now and your DC is under 2, your body might not have replenished its iron stores before you became pregnant with your current baby. It might be worth getting tested, if you haven't already. It's worth bearing in mind that the reference ranges are wide, and everyone is different in terms of the iron levels they need to function well, so even if the doctor says your levels are okay, they might not be okay for you. Vaguely remember a study where healthy non-pregnant women with supposedly adequate iron levels were given iron supplementation, and after a month, most of them had experienced something like a 50% reduction in fatigue...and these are women without tiny personal lodgers stealing their nutrients!
I am 37 weeks pregnant with my first baby and I've taken Pregnacare all through, but I think what's made the difference in terms of keeping energy levels okay is Floradix, which is a liquid iron supplement that's gentle on the digestion but very easily absorbed...more so than normal tablet-type vitamins so I understand. (added to which, I've read on MN that the ones GPs prescribe do dire things to one's insides and are to be avoided if at all possible)
So Floradix is basically magic mouse serum. I've had good iron blood results from the get-go, and on the occasion I ran out of the stuff and forgot to get more, I really noticed tiredness kick in about a week after. It's safe to take with Pregnacare - I emailed the company to check...the only downside is that it is fiendishly expensive and only available from Holland and Barrett, not Boots (so no Advantage Card points...booo), though H&B do sometimes do Buy One Get One Half Price offers. I haven't tried Spatone sachets, but if you do look into iron supplementation it might be worth asking other MNers what experiences they've had with that.
Obviously your iron might be totally fine and this tiredness could be any number of other medical things (thyroid etc)...but it could also simply just be Life telling you to be gentle with yourself and not to try and be Wonder Woman whilst growing a human from scratch. 
I take my hat off to you for managing to be PG, go running, work 4 days, with a hefty commute and to keep all the plates spinning...all with a DC already... 
Very best of luck with your pregnancy.