This was the second marathon road course I've run (after NY) -- while the course was nearly flat, it was challenging because of:
Lots of people going through many narrow sections,
Curbs you could step off of wrong if you were not watching your feet (I partially rolled my left ankle in the first mile, rude awakening. Felt it for the next 10 or so)
The warm weather (mid 60s and sunny, the sections in the shade were pretty pleasant but in the sun you cooked!)
I wasn't prepared for this race in general (signed up for it a week or so ago), and wasn't optimistic about even finishing (the last month had seen no runs over about 5 miles). I positioned myself between the 3:40 and 3:50 pace group balloons, figuring I'd start by running 9s.
For the first half (and after my ankle rollover especially) I just hung out with the majority of the runners I was near, running 9s as planned. I didn't see too much passing, but maybe it was subtle? The first half went in just about 2 hours, and a bit after I stopped to stretch a little, massage my right calf (which had been getting tight on longer runs causing tendonitis in my ankle, which is why I hadn't done any long runs in ages), and my left ankle seemed to be fine, so I kept on. Around mile 20 I realized that a) I was going to finish, and b) I was actually feeling pretty good considering (my HR had been 130-140 or so the whole time), so I decided I'd run some 8s when the crowded course allowed and get some of those newfangled negative splits, which was successful.
Seeing Al and friends on the course was awesome.
Swimming in the harbor after was incredibly cold and perfect and 10/10 would do again.
Aid stations have signs ahead of them that say "OPEN BAR 100 meters ahead" which I find hilarious.
Aid stations use a ton of plastic cups.
I carried a backpack with me which turned out to be handy (extra water, ended up having a liter or so -- the aid stations were few and far between and as a result crowded and difficult.. but even if I stopped and had 2 waters + 1 energy drink at each, I would've still finished very dehydrated).
If you (like me) are not too familiar with Copenhagen, you don't really notice the loop in the course, because by the time the second time comes around you're in full "make this shit happen" mode.
The dry-bag backpack that they gave everybody is a fantastic perk, especially given how affordable this race is (compared to NY at least!).