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?

Hope this make some sense, thanks.

Ok, let me put it another way is it necessary to continue with LSD rides once you start to up the intensity during build periods?

Especially when you ride marathon races I would keep one LSD session per week, except in race and tamper weeks. The LSD doesn't need to be longer then your races. But I you can do the LSD ride 1-2 hours longer then the race, you will get the pay back in the race.
Even with a 3 week tamper I will still do 1 LSD ride at the mid point of the tamper.

db, thank you for your reply and the input. I'll continue with the long rides but up the intensity to include some long steady Z3 tempo efforts instead of pure Z2 LSD type riding.

My answer is very generic. It highly depends on your short and long term goals. Not only for this year but future years. How long are your races, how many. What are you strength and weaknesses. And most importantly: what do you want to improve?
But I'm still convinced that if you cut your long ride, you will lose endurance, but not strength. I have a hard time keeping in zone 2. Its often high z2 / low z3 when I'm done

last season I found quite efficient to include some hard work in my LSD ride (4 hours or a bit more) like 2 x20 or 5 x 5. Tempo efforts also work great for me

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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.

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Norwood & Adelaide Uni CC

Last year I followed a pretty standard endurance training model and for 2-3 months I trained at low intensity, but I quickly discovered I couldn't get anywhere near enough volume to promote any significant changes, apart from weight loss. That's why I chose a different approach for this year, increasing the intensity so that most of my training time during base was mainly SST, tempo(low end Z3) and usually one long 4-6hr LSD ride. Now during my 5th week of build I am getting around 10-12hrs weekly which allows me some pretty quality training.