Mach 2 is roughly 2400 KM/H. This is faster than most of the planes in the game can reach. In fact, the only 3 planes I know for certain capable of reaching and sustaining that speed are the YF-12, Mig-25 Foxbat, and the EF-111A Raven. Some other planes are capable of flirting with that speed during a steep dive at full power, but often times they can't maintain it.
Your best bet is probably to join a co-op game using the YF-12 or the EF-111A and keep it at full throttle until it finally unlocks. Hopefully you won't piss everyone else in the game off in the process. If people do start getting frustrated with you, defer to using the EF-111A. At the very least it is capable of carrying a few weapons so hit and run strikes are reasonable at high speeds.
Theoretically you could just jump in Free mode and fly around for an hour, but chances are you'll go nuts in the process.