Hi, I’m a hobbiest cyclist . Mountain bike in winter along with indoor training and mainly road bike in summer. A couple of questions. I have a sportive one April around lochness in Scotland, nothing major, a 66 round trip which I have done before. Not a race just a sportive for enjoyment. I need targets for motivation , should I use plan builder and add this even as my A event, or would I benefit more from just the standard base, build speciality programme that I have used before?? Next question, how does mtb time compare to road bike time. As in if I’m mountain biking for say 3 hours half up half down , pretty long hard climbs, how would that compare to say 3 hours at 16mph average rolling road? What I’m trying to ask is can mountain bike ride time , equate to similar on roadbike although a much shorter distance??
I think TR did some podcasts on the subject basically if you’re event is on the road train for the road. That being said time and conditions (especially in Scotland) may not allow efficient outdoors road training and doing something is definitely better than nothing. You can certainly get a good workout in on a MTB in a shorter space of time. I’m not sure if anyone has put an exact figure on it though 
I do a lot of both road and MTB and here is my non scientific way I think about it: Both my MTB and roadie have power meters, and I find that on my typical terrain (New England, so rolling hills, no massive climbs, rocky and rooty off road) that my AP will be 20w less on the MTB, usually something like 200 v. 180. The NP will be closer due to the punchy nature of MTB. Given I’m using a PM, I can look at the TSS and get a decent idea of the value of any given workout.
Translating that to non power meter terms, I would say from a time standpoint an hour MTB ride is pretty much equal to an hour road ride when you account for the lower AP/NP but greater ‘whole body’ use of MTB. From a distance perspective, I would say a MTB mile is worth about 2 road miles. These equivalencies are going to be terrain dependent, but I’m sure you could come up with something similar that fits where you ride.