It's not so important to use a specific type of bag, just make sure it's comfortable, and a great option is a bag with a water bladder, so you don't have to carry a water bottle. You're ideally looking for something lightweight, suitable for carrying water, energy gels and an extra layer, so a hydration pack will suffice. The most important part is the straps. The shoulder straps should be padded and wide enough to distribute weight evenly, the chest strap should be able to be moved up and down, (especially important for female runners), and all the straps need to be adjustable.
Hydration pack
You'll see many runners wearing a running vest, sometimes known as a hydration pack or race vest.
These are extremely lightweight backpacks with multiple pockets and pouches to carry everything whilst being easily accessible. They come in different volumes and sizes, and should fit what you need when out running depending on whether you're out on a training run on the local trails or a multi-terrain full-day of trails or even a race event with a lengthy mandatory kit list. Items you pack may or may not include water/fluids, snacks, phone, keys, hats/gloves, spare kit, headtorch, sunscreen etc. Like buying running shoes, no one pack is perfect for every run. Before you buy try it on. It should sit high on your back with shoulders and upper back taking the weight, leaving no pressure on the lower back. It should be snug enough to stay in place, but not be so tight that arm movement is restricted or you feel anything rubbing around your neck or shoulders. When the chest and tummy straps are fitted well, the hydration bottles or their pouches should lie vertical and not at too much of an angle. Most brands offer gender-specific models, some just unisex. Women's-fit packs tend to be narrower at the shoulders and offer more volume in the chest area.
Bottle vs reservoir or bladder
A personal preference thing. Some prefer to carry bottles stowed in front pockets, whilst others prefer a larger reservoir that fits into a sleeve in the back. Most vests are compatible with both options, but bladders aren't usually included in the purchase of the vest.
- reservoirs: hands free, fuss-free, carry more fluid
- bottles: more versatile, easier to refill, but can be a fuss getting in and out of the pouches whilst on the move, some come with a drinking tube
Brands to look at are Salomon, CamelBak, Adidas, and Montane.