>Cross Training>Push ups and back pain
Roads were made for journeys...
As people always say with running, just do less of them.
It sounds like your lower back is the weak link, so do half or a third of the amount of core work you were doing, and build up.
It works with any physical conditioning workouts, not just running. I was talking to a guy I know who does powerlifting a few years ago and was amazed at how similar the training periodization cycles were.
"Today I broke my record for most consecutive days lived!"
Run until the trail runs out.
The pain that hurts the worse is the imagined pain. One of the most difficult arts of racing is learning to ignore the imagined pain and just live with the present pain (which is always bearable.) - Jeff
Alternately, you could do knee push-ups, which although easier on the arms are also easier on the back.
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