Cross training means doing non-running activities, like cycling, swimming, or strength work, to build fitness while reducing the repetitive impact of running. It helps runners stay fit, manage fatigue, and lower the risk of overuse injuries.
Common options include cycling, swimming, elliptical work, rowing, and strength training. The common thread is that they build cardiovascular fitness or strength without the repeated pounding of running.
One to two sessions a week works well for most runners, usually on days between runs. It's most useful when replacing a run you'd otherwise skip due to fatigue or minor niggles.
Indirectly, yes. It builds general fitness and strength that supports running, though it doesn't replace the specific benefits of running itself.
For one or two sessions a week, yes, especially during a niggle or a heavy training block. It shouldn't replace most of your running, since race-specific fitness still comes primarily from running.