In reply to Outershell:
If you were capable of doing Tree Traverse at Pleasley, you were already strong enough and fit enough to onsight 6c+ while putting the draws in.
I know this doesn't answer your question, but something else must be holding you back. Do you maybe have a problem with fear of falling or overgripping or bad clipping technique?
Identifying what the issue is and working on that will probably yield better results than more endurance training. Ideally, you could do both.