If you're planning on running long distances or heading into the woods for your run, you may want to consider a running backpack that will allow you to store food and an extra layer of clothing should the weather turn bad. Many running backpack models feature a hydration system that includes a bladder and hose; this makes drinking water on the run much easier, but the system can also add cost to the pack. To save money, you can buy a pack that features water bottle holders rather than a bladder system, but it will not be as easy to pull out the water bottles and take a drink.
Choose the right size running backpack by determining what you will be bringing with you on your runs. If you will only be stowing a small bit of food in the running backpack, a very small model will do just fine. This will prevent you from having excess weight on your back during the run, which can lead to discomfort. You may even choose a running backpack that features a bladder only so you will have drinking water and no extra storage space.
If, however, you intend to bring an extra jacket, some food, and maybe car keys or a wallet with you, a larger running backpack will be in order. It will be important to choose a pack that fits correctly, especially if you will be carrying a fair amount of weight. Try to choose a pack that features adjustable shoulder and waist straps so you can find the best fit. A waist strap is particularly important, as this will prevent the pack from shifting up and down as you run. Some of the best packs feature unpadded mesh straps for better ventilation, though others may feature padded straps for added comfort. You will need to try out both types of straps to figure out which ones work best for you.
Some more advanced packs feature mesh backs that prevent the pack itself from pressing against your back as you run. This will help prevent sweat buildup that can lead to discomfort or even rashes as you run. If you plan on running in hot conditions, this may be a worthwhile investment, but keep in mind that such a running backpack is likely to cost more than other models. As always, fit is the most important consideration, so if the pack you like does not fit properly, do not purchase it.