Russian forces walked inside a gas pipeline to strike Ukrainian troops from the rear in Kursk

At 2mph it would take 4.5 hours. A regular SCBA bottle holds about an hour, depending on physical exertion and tank capacity. Even assuming they have ones that are better than what you or I can buy, that's still an excessive number of tanks per person to haul, plus their other equipment.

Cutting holes to recharge is possible, but time consuming and risky. Pulling hose for fresh air isn't feasible at that distance. Just blowing fresh air in the pipe might work, but it'll take a big lead time to push it ahead of them, and isn't effective if they can't push the bad air out the far end.