1,500 miles? Surely there will be bases and depots that are much nearer. Doesn't have to come from the USSR., as long as railways are kept going it not a big deal. There we be 10,000's of men to fix them, even at gunpoint.
Actually it is a huge deal, incredibly destructive to ongoing operations on either offense or defense.
The WAllies ran into significant issues within a few hundred miles of the French coast due to fuel starvation. They eventually solved the supply issues with a mainly above ground pipeline, but it wasn't really resolved until the took Antwerp and reduced the length of the tail, and that was without having any jackasses hitting their supply depots with 300 or 400 plane air attacks (with each aircraft carrying five times the bomb load of the B-17/B-24 (or about 10x if the B-36 has entered service) the WAllies also will have clouds of escorts, but late model P-51s and P-47s as well as P-80C (at least as far as Berlin), F-82, F6F, F4U, and F7F (after the initial clashes USN Air is going to be sort of at loose ends, freeing up thousands of fighter/bombers). Baku itself in within ESCORTED range of B-29s and Lincolns flying out of the Middle East/Crete/Cyprus (all of which are well out of range of Soviet air power)
Soviet forces would need to transport fuel from the VOLGA or out of Romania all the way to the front. Weapons and munitions, in large part, from the Urals. And would have to do all of that without a reliable source of spare parts for their overwhelmingly American (via Lend-Lease) trucks. Keep in mind that these are early 1940 engines, the sparkplugs and points last, if you are lucky, about 5,000 miles (God knows I changed enough of both on a 57 Chevy truck engine in my misspent youth). It is really very difficult to "fill 'er up with Champions" when the single supply point is in a shooting war with you.
Every mile of that supply line is vulnerable to WAllies air attack. Every Studebaker or Ford that gets knocked out is literally irreplaceable.
The Soviet do great right up to when reality grabs them by the vertical smile and won't let go.