Those afflicted with HPDE/track day addiction are destined for a frustrating existence.
Once you're neck-deep in the hobby, there is no substitute. Problem is, track driving is also one of the least accessible hobbies to have. Most don't have a race track that's readily available to them, and the running costs for items like track tires prohibit really consistent participation anyways. Oh, and everyday life tends to get in the way as well.
Given the limited accessibility, methodical improvement and advancement is a challenge. With every new track day comes a re-acclimation to the experience. You don't simply head out on track and from session one pick up where you left off weeks, or even months ago. It takes some time for the body and mind to once again acclimatize to the very unique context.
We previously discussed how modern video games, many of which are a very realistic simulation, can help the cause. (See Can video games make you a better track driver?) But ultimately no matter the realism, you can only get so far whilst sitting on the couch.
Mountain biking may be the most effective cross-training activity for performance driving. Singletrack and/or downhill mountain biking, more specifically.
The parallels are numerous:
Mountain biking promotes momentum awareness and development. Carrying maximum speed, and maintaining momentum is an advanced HPDE technique. In mountain biking either you'll learn the skill of momentum, or your legs will suffer.
And mountain biking promotes reflex speed as well. The closest we've come to simulating the speed of response required to catch an unanticipated oversteer moment on track is keeping upright on the bike after losing traction.
Understanding weight transfer and how to load the tires is common to both biking and performance driving. Getting either vehicle to behave as desired around a corner requires the appropriate weight transfer and balance. Techniques like trail braking can be applied in both disciplines as well.
In singletrack and/or downhill mountain biking, the rider is forced to make ongoing and precise steering adjustments. The very measured steering inputs to the bike's handlebars are similar to the measured and controlled inputs required to drive smoothly around a road course.
Last but not least, mountain biking builds endurance of the sort that is useful in track driving. The body is stressed in an uncommon way on the race track, and an ordinary conditioning exercise like jogging won't prepare you for it. While not equivalent in terms of g-forces, mountain biking approximates the stresses associated with cornering forces and speed in general.
And as a cross-training activity, mountain biking has the added benefit of being seriously good fun. Like we said, there is no substitute for a track day, but we'll take a singletrack mountain bike ride as a first alternative.
For more cross-training ideas, check out Nico Rosberg's regimen in this video. Although… you really can't get away with this sort of stuff in a public place unless you're an F1 driver.