Ok, you guys are probably going to laugh at the noob but here goes...
I've recently started riding a 'proper' (well, a CX) bike on the roads instead of my old hardtail MTB. I've gone from platform pedals where I'd usually just monster everywhere in a high gear, to now using SPDs and trying to keep a steady cadence by using my gears more.
Why am I getting so exhausted? I get that pedalling with SPDs takes a bit of getting used to, and I get that there will be a bit of transition period while I get used to the new bike. It accelerates from a standstill far faster than the MTB did (probably due to lack of suspension) and flies downhill but the slightest uphill gradient and I end up crawling along at a ridiculous speed.
I know the problem is me, but what exactly? I can't pin down the main thing that's different, because everything feels different. I just know that where as previously I'd just mash away with my quads (destroying my knees in the process) to go X speed, I'm now using a lot more effort and concentration to go X+not a lot.
Help