For first time marathoners or half marathoners, you don't want to have too long of a stride or be going too fast. You have 13.1 or 26.2 miles to get through so your pace is very important. Some tips I've gotten over time that I want to share with you are these:
1) Keep your hands relaxed you shouldn't be clinching them tightly pretend that you are holding an egg so close your hands softly but not have them closed all the way. Again this reduces excess energy waste (again you'll be running anywhere from 4-6 hours so little things matters)
2) Your fists or arms should never be in full sight of your eyes when you swing them upwards. If they are, they are too high and you are wasting energy by taking too long of strides. Instead if you are looking straight ahead try to get them to just come into your peripherals of your eye site.
3) Remember your pace, don't swing your arms too fast as your legs will follow so keep it at a comfortable swing and if anything I think swinging a bit slower is better early on as you can always go faster later.
The key to this is energy conservation, remember you have a long run ahead of you so all these little things add up especially towards the end, you want to have as much energy as you can with as little waste as you can.
Happy Running!
