To elaborate more, if you are a cyclist taking up running, you will probably have a well-developed cardiovascular system but not the required strength in the musculoskeletal system to put up with the impact forces of fast running. Just like a newbie cyclist, start slow and build up distance first before intensity. Your body will slowly adapt to the requirements of running but only over time and only if you give it enough recovery.

The most I personally could sustain was both about 60 miles running and 130 miles cycling in a week. Once I started getting close to over 15 hours training per week, it was too hard to keep up with a full time job and my sleep habits. If I was only cycling, I think I could probably handle 20+ hours/week, but when only running I've usually only maxed out 12+ hours/week before I started to get dead legs after a few weeks. I'm not hardcore enough nor do I like chlorinated pool water enough to be a triathlete...