Typically it's one's cardio/aerobic system that is the limiting factor - improvements in this system will come with riding. Lifting contributes to cardio fitness, but it's generally anaerobic, which as endurance athletes we try and avoid until absolutely necessary. Lifting will contribute to general fitness, and prehab making you less prone to injury. It also aides in flexibility - routinely doing deadlifts will increase flexibility in the hams and glutes. But it won't necessarily make you faster, with the exception of anaerobic efforts like sprints or climbing.