This very scenario was brought up in the article The Storm and the Whirlwind, from the Peter Tsouras book Third Reich Victorious.
Basically, the Luftwaffe picks up on Russian preparations, and that combined with the Russians cutting off shipments of goods and materials lets the Germans know what's coming.
The Russians make some initial progress, but the Germans then counterattack and perform a huge envelopment, encircling and destroying or capturing much of the Russian forces on the central front.
When the dust settles, since the Russians front-loaded the attack, there's nothing between the Wehrmacht and Moscow but a few undermanned and underequipped infantry corps. A month later, the Wehrmacht has taken Moscow, Leningrad, and Stalingrad, and the Russians sue for peace, which is truly draconian.
Written by somebody who hasn't read the list of Soviet (yes, Soviet, not "Russian") that couldn't be "front-loaded" because they had not been mobilized.
There is a workaround, of course: if the Soviets are planning this since 1939, then those forces will have been mobilized, and might be used in the initial attack. This is unlikely, because the Soviets really were pretty convinced they were relatively safe throughout 1940 (first because the Germans were occupied elsewhere, and then because moving to the East would be too lat ein the season). But it might happen.
I have my doubts, though, that all the new mobilized forces would be thrown in battle immediately: the hard limitation on how many troops the Soviets can "front-load" into the opening attack is exactly their poor logistics. Even if the Soviets do have ready by spring 1941 all the troops that they in OTL had plus those that they mobilized or raised later, a sizable part of this humungous army simply can't get on the first trains to the frontline. So the Germans may encircle and destroy the first wave, which will be more or less the same troops they did destroy historically. And we're back to the rest of the Krasnaya Armiya.