JOIN takes into account your preferred number of runs per week but doesn’t necessarily reach that number every week. As you are accustomed to with JOIN, JOIN only plans a workout when you are well-rested enough to improve by the planned workout. Sometimes, it’s just better to take a rest day, for example.
JOIN also gradually builds up the duration and intensity of your runs. Thereby, it also takes into account what you have done on the bike. It doesn’t make sense to get a cycling VO2max workout the day after a VO2max run. Of course, JOIN also takes into account that running is, in general, physically more demanding than cycling. This is not necessarily the case on a cardiovascular level, but it is in terms of strain on the muscles and impact on the ligaments and joints.
In general, runs are, therefore, shorter than rides. But you can get pretty long runs when you have shown, based on your training data, to be physically fit for that. Of course, JOIN will never plan a run longer than your availability. So, if you want a shorter run than prescribed, you just need to adjust your availability.