(photo pulled from Wikipedia's article on 29ers)
So the mystery of why my last few rides have felt so strong has been solved.
Riding my "old faithful" 26" singlespeed the last few times out, I was making climbs I rarely (if ever) make. It turns out the reason is my gearing. It hadn't occurred to me that the 32x18 gearing on my 26er was an easier gear than the same 32x18 I run on my 29er.
Last summer/fall when I was riding more, I didn't really notice the difference. Probably because I was in somewhat better riding shape. After taking the winter off to get married and house shop and uhh...drink beer and um...watch TeeVee, I notice it now. Last summer/fall, I had been riding my 29er regularly and had gotten used to the tougher gearing. When I picked up my 26er for my recent rides, I was running an easier gear.
I took the 29er out on Tuesday and it felt like I was trying to climb those same seldom-conquered hills with tires mired in oatmeal. Granted, it was the first time I'd ridden on dirt two days in a row in a long time. So that didn't help. But I think the main performance hit(s) was(were) the gearing and the big 29er wheels. The bigger wheels and tires have more weight/mass which require more power to get turning. The added momentum of that weight once it was spinning was nice. But only spins for so long on the climbs.
As for the gearing...According to Sheldon Brown's high-tech gear calculator, it seems that a 32x18 on 29" wheels gives a gear-inch measurement of roughly 51.4 while the same 32x18 on a 26" wheel gives you a gear-inch measurement of roughly 46.1. So if my limited understanding of gear-inch measurements is correct, I should be running a 32x20 on my 29er, which will give me a gear-inch measurement of 46.3. Much closer to what I'm used to. Looks like I'm going shopping. I'm sure DT will be sad to get that fancy Boone TI cog that I've been borrowing back.
Special thanks to DT for pointing out the differences in gearing between the 26er and 29er.