I race and ride mtb XC marathon events and having spent most of the winter riding 1-2 hr Z3 tempo rides, SST 3x20's and LSD road rides 5-7 hrs I am now half way through an 8-10 week block of threshold training i.e. 2x20's and some 60-90min >Z5 interval type sessions (various) and a 2hr SST ride 3x20's along with usually an additional 1-2hr Z2 road ride and 1 long weekend XC ride 60-80miles 5-7hrs which is dependent upon the weather and life in general, if this is not possible I'll hit the trainer for another SST session.
Question, is this sort of structure enough to maintain a decent level of endurance?
I'm training for the oz marathon mtb series at the moment and am finding that averaging 11.5 hours is working well perfectly well for endurance. My races are either week long stage races or one day 120km marathons - I tried a similar program to you last year and I found that I wasn't nearly fast enough to finish where I wanted.
Had 2 months of 5 x 3.5hr LSD rides to establish base. For the last 1.5 months of lead-up to a 5 day enduro (late May) that I have, I've switched to 5 x 1hr turbo sessions + two 3hr rides that are ~ 75% of race distance. Am getting really fast compared to previous training models - and able to keep it up for 100km +, which is also new.
Would help to know you base background in the last year.