many other sports besides running to compare
In my younger days I spent many many hours playing basketball, handball, racquetball, softball, etc. Ran a few 10K's but running was not very interesting to me. As I got older, the knee, leg & back issues in the sports were increasing but nothing like the impact running had on me. Step aerobics 3 - 4 times per week for a decade were my last regular exercise routine until we started cycling last year. Yoga, stretching, a few weights & cycling have provided me with an exercise routine that is less painful re: joints & manageable re: musculature & stretching.
So as a Clydesdale, I cannot relate to 'giving up' running for cycling but the hand/eye coordination of offroad cycling & the roadwork of pushing that bike up & down our long gentle slopes provides plenty of challenge. CrossChain & 88 are right: it is as challenging a fitness regime as you choose to make it.
As a note: each of my fitness periods seemed to produce a 'different' body over the months - the court sports, step aerobics, Body Pump with light weights & many reps & now the cycling all challenge my body in different ways.