Well, from my point of view (former road cyclist) and now Sky and Vertical runner, cycling always provide me big aerobic capacity. When I want to do some very long endurance in zone 2 with a lot of vertical meters than I would always prefer bike. Running for more than 5 hours while constantly going or up or down makes my legs pretty tired (however, I am doing long endurance also by foot, mostly up to 4 hours).
If I want to be consistent, that means train day by day, integrating aerobic and anaerobic stuff, mixing endurance and intervals, I should do like that. For example after 5 hours on a bike I can do next morning interval session (30/30). However, this is just my point of view. I am not ultra runner and for doing ultras I agree, you should do little cycling and rather prefer running on trails.
Last but not least, I also use indoor bike for some quality as well. Doing max sprints for 10 seconds for example and than 2 – 3 minutes the spinning the legs. If it suits you, if you are used to do … don’t hesitate to keep on. Go out of the box, explore and learn from the mistakes. 🙂
