I'm interested in using spinning (group stationary bike class) as part of a long-distance-running training plan (from half marathon to ultra marathon). Typical plans include running at least 4 days/wk and weekly mileage of 20mi to 50mi (or more). I've completed marathons before without including cycling (or much cross-training) but I want the added benefit that comes with cross-training on days off from running (avoiding injury and adding fun are also motivating factors).
It should be given that I go easier/harder based on feel, I do not spin the day of a long/slow run, and that there are day-to-day variations (e.g. the instructor and their plan for the day, the bike itself, etc.).
I'm wondering how to choose the right level of intensity and whether it is more beneficial to focus on cadence (speed) or resistance. Or is the random nature of this form of cross-training the main benefit? (I'm thinking this is similar to the fartlek run style of training.) Lastly, classes are 1 hour long - is it wise to let this type of training actually supplant some amount of running? i.e. each week let N hours of spin reduce running by N hours (up to a point).