There is two schools of thought about incorporating cross training into a running program. The first school will say only running improves running, however more research is showing that cross training including bike intervals and strength can improve times over distance events.
Some benefits: maintain fitness for running without the repetitive loading. It provides a variety in a program, to avoid getting ‘flat’ spots or demotivated. It is essential whilst injured to maintain cardiovascular fitness and strength.
We recommend: cycling or elliptical machine. You can include a 2nd quality session to your week by doing short efforts up to 1min, in a high gear.
Water running, walking and uphill setting on a treadmill can assist with boosting up volume, and over time can increase the mileage in your running.
When? It can be good , if aerobic to do the day after a hard session or if it is hard then do with a day between your last hard run session.
It is best to avoid too many gym classes that build muscle mass rather than muscle endurance. If your goal is to run faster then keep it specific. If your goal is fun and fitness then running plus classes can be a great way to maintain the variety week in and week out.