I also remember "Wings" documentary on the Discovery channel- it mentioned that "after a unit of new MiG-31s was sent to Kamchatka, the number of recon flights near Far East Soviet airspace reduced drasticly". AFAIK, the Soviet recon aircraft didn't come as close to the US airspace as the American ones came to the FSU airspace.
Wings is not exactly a useful reference - it's a video version of Wikipedia at a QA level.
The frequency of US ferret runs and overflights is directly linked to technology shifts in the US
ie post ww2 to 1948 was using B45 Tornados - awful tech, but useful until the russians got their Nene modified jets in the air
1948-53 it was via the RAF using EE canberras - and then modified hi-alt canberras as the soviets couldn't reach them. so they went lo alt and then high alt within 5 years
1954-59 the US used modified licences built canberras - again hi-alt mods
also the use of U2's as a purpose built hi-alt solution. the first sub-atmospheric overflights
1959 was the start of mach 3 overflights to counter russian GBAD after Powers was shot down. Never intercepted once
in 1972 hi-speed overflights over russia were pulled as keyhole and its brethren came into play. US hi-speed overflights were restricted to Vietnam AO
basically every mission after the late 70's was a satellite job. reason? 10 times the coverage in longer pass and continuous overlap available if the constellation was timed.
the Mig31 never impacted upon overflights as the US had made the decision years before (a decade before) to run with satellites where the images were easier to take, pilots were not at risk, and where more extensive footage was able to be harvested.
so, Wings is way off track (by nearly 10-15 years)