What makes mountain biking hard? Well, a little of everything. SuperFan Cindy and I didn't arrive too late, but late enough that my warmup lap meant going right to the start queue and having just a few minutes before race started. That might be considered even good timing I guess. I wasn't sure what legs I'd have. I'm a) trying to de-fatten myself so I've been eating less, and b) I did my telemark leg exercises Friday as a bit of a rebuild for doing some coached skiing next weekend, and have the second day soreness going.
The start was fast, I sat in 2nd last. The first long descent seemed slow, but I was content to sit in and see how everyone's endurance was. The climbs were hard for me, but I survived with the gearing I had... a series of issues yesterday (seized then stripped cassette lockring, cutting my hand when it stripped, trying to just change it over to this PowerTap wheel I had, etc.). Eventually I put on a 10 sp mountain bike cassette that's got a 36t large cog. I made it today on the 32-36, but I've been riding a 32 up front and am used to the SRAM XX1 42 in back.
Faded on the climbs on last lap and I ended up 6th. Which I guess is fine for me actually.
So other than that ancillary stuff, back to the question - what makes mountain biking hard? It challenges all parts of your energy output ability:
Secondly, this one did have a reasonably defined section of downhill to rest, but the two climbs and the "flat" headwind back to the start finish had nowhere to rest.