Best setting for running without gps?

I think your best option, from what I read, is to get one of those Stryd Sensors.

Stryd is overkill, any ANT+ footpod will do.

Without gps, the watch does a bit more than count strides, and it calibrates the internal accelerometer across some range of paces. I’d be inclined to test it out on a route of known distance before you decide if you need a foot pod. 

The Stryd foot pod handles a range of paces much better than the Garmin pods do in my experience. The Garmin ones were fine at the pace where I’d calibrated them, awful at significantly slower paces. My Stryd is consistent but off by 1-2% all the time, depending on the shoes it’s on. Better than gps where you have poor sky view and sharp turns, a wash under better conditions. If you’re going to run at a fairly consistent pace, a cheaper pod that you’ve calibrated at close to that pace will be just fine. 

Not sure if the Race Screen data field would work without GPS, but if it does it's the perfect option - since I guess you will have km indications on the way during the race :)

I would be more worry about running a half marathon inside a tunnel ... LOL
Unless you are working inside iron mountain or a nuclear submarine ... :D

As mentioned by mcbadger, a regular foot pod will do the job under normal pace conditions once calibrated but the Stryd will handle various paces and running conditions much better without calibration (in most situations).  You also get more data from the Stryd but whether you need more data is something only you can answer.  Obviously also comes down to the significant difference in cost.  If you figure you will have a relatively steady pace and the opportunity to calibrate then a basic foot pod might be best for a one off.  If you think you can use a foot pod for pacing in multiple situations the extra cost might be worth it. 

Well, I guess there will be markers every 1 km. Is there any way of just setting the laps manually? I have tried turning off auto-lap, and setting laps manually while I run. But my watch still uses it's own(And wrong, by over 20%..) calculated pace in applications like Strava...

I mean, maybe I could just use the stopwatch function and log every 1k. Just would have thought there could be a better way without buying any more sensors.