Assuming you had access to a power meter and a heart rate monitor. What sort of training would you do?
To just turn it over for a short period? You would just need to have above-average leg strength: the quickest way to gain this strength would just be to do leg strength exercises such as squats, leg presses, etc. You don't need superhuman levels of leg strength merely to turn over the gear.
The hard thing is to turn it over at a good cadence, for an hour at a time. If you don't have the cardiovascular capacity, your legs will be burning and you'll be gasping for air in short order.
To actually make use of the gear you need to have the cardiovascular capacity of be a world-class time-trialist. The first step there is to be one of the few born with sufficient innate ability. The second step is to do years of intense training building up your cardiovascular fitness. After that, you would probably need to work out what your preferred cadence is at time-trial speeds. If your cadence is slow enough to need the bigger gear, then you probably already have the leg strength needed. If your cadence is fast enough that you don't need a big gear, then why bother?