Russia doesn't.
I disagree. The “length” of a supply line is not measured in just distance though that is one of the factors. For example, assume the same two cities for source and destination. A supply line by rail is preferred due to the ability to move any cargo, regardless of composition or size. The same supplies delivered via air is much more complicated (considered “longer”) as transport via air is more difficult and often consists of smaller loads that have to have a certain amount of favorable weather.
Ukraine has demonstrated the ability to attack Russia’s ground supply by attacking convoys and routes/bridges. Russia has not yet demonstrated the ability to consistently and effectively attack Ukraine’s supply lines. However Ukraine has to ship from NATO countries.