Search "Shermer's Neck" a condition named for Ultra marathoner Mike Shermer for some insights.

I'm not saying you have this, which is related to days and hours but the causes are related.

Eventually your muscles will strengthen (hopefully) and the amount of time you can go will increase, but in the meantime, you want to prevent the muscle spasms holding your head up causes. There are two basic methods that help - short of changing your riding posture entirely.

1- keep your head and neck in motion. You want to avoid the dead ahead stare that is causing the spasm, so become a sightseer. Look up at the birds, down at the pavement, and to both sides and enjoy the scenery as you pass. Of course you also have to look ahead, and sometimes behind for traffic, but the key is to keep your head and neck in motion as much as possible.

2- reduce the load on your neck by minimizing the amount of time you look ahead. So look up and check that the road ahead is clear, then you're free to relax your neck and let your head drop for anywhere from 5-20 seconds. Then repeat. Of course, this is not suited to busy urban roads because too much is happening, and there may be surprises at any time, but on open roads with long distances between intersections, you could ride for hours while actually holding your head up only for minutes, which will make a world of difference for you.

Between the two methods you can make your riding life better right now, while you wait for the improvement that will come over time.