Vest vs Backpack: A Real Distinction
A race-day hydration vest, covered elsewhere on this site, is built around soft flasks worn close to the chest -- light, low-bounce, and sized for the gaps between a race's aid stations. A hydration backpack is a different tool for a different problem: long, unsupported training runs where there's no aid station at all, and a runner needs to carry everything -- water, nutrition, a phone, maybe a light jacket -- for the full distance.
In Southeast Asia specifically, unsupported long runs are common simply because a lot of training routes (park connectors, coastal paths, longer neighborhood loops) don't pass reliable water sources the way a certified race route does. That makes backpack-style capacity genuinely useful for serious marathon training, not just a nice-to-have.
Capacity: Matching the Bag to the Run
A 15-liter trail-style running bag with a 2-liter water bladder covers most long training runs comfortably -- enough water for 2-3 hours in hot conditions plus room for gels, a phone, and a light layer, without the bulk of a full hiking backpack. This sits deliberately between a race vest (too small for genuinely unsupported distances) and a full trekking pack (more capacity and weight than a training run needs).
Sizing up further only makes sense for genuinely long unsupported efforts -- think ultra-distance training rather than standard marathon prep -- where the extra capacity justifies the added weight and bounce of a bigger pack.
Fit Matters More Than Capacity Once You've Chosen a Size
A poorly fitted backpack bounces with every stride, which becomes genuinely uncomfortable and can chafe over a long run -- adjustable chest and waist straps that let the pack sit snug against the back, rather than swinging loose, matter more than any other single spec once capacity is sorted. Trying a pack on with some weight in it (even just water) before committing, rather than buying based on capacity numbers alone, avoids this problem.
A water bladder with a wide-mouth opening is worth prioritizing too -- it makes adding ice on hot training days, and actually cleaning the bladder between uses, meaningfully easier than a narrow-neck design.
Beyond Water: What Else to Carry
For a genuinely long unsupported run, a hydration backpack's extra capacity is also where sun protection, a phone in a weatherproof pouch (both covered in this site's other guides), and enough nutrition for the full distance actually fit -- a race vest's minimal storage doesn't have room for all of it at once. Planning what goes in the bag before a long run, rather than packing at the last minute, avoids discovering a gap (like insufficient nutrition) partway through a route with no aid stations to fall back on.
For training routes that do have some access to shops or water fountains along the way, a smaller bag still makes sense -- the backpack is specifically for the genuinely unsupported stretches, not a default upgrade for every long run regardless of route.