I run as well. I have done a few duathlons. That is what got me started into biking.
I would suggested a run/walk regimine when starting for sure. Be aware it may take a few months (3-4 for me) to get your body in shape enough for it not to be so torturous and be able to do any distance that really starts burning the calories.
I have been down for 3 weeks due to a ankle tendon problem. It is really starting to get annoying..
Anyway, I like the crosstraining. It keeps it from getting boring and the running burns more calories per minute than cycling and it is a much harder cardio workout for me and I suppose for most others as well.