I've taken a 15 month old. He did perfectly fine on all of the rides he was tall enough to ride. I did like using my Ergo-style carrier (I use a Sutemi, but they're almost the same), but I didn't need it for the rides themselves. I liked it in lines so that I could carry him much more easily than carrying him in my arms. He'd started walking at 13 months, and he was still likely to drop down and crawl while in lines (yuck!) or want to stop and look at things when we were going through ride queues without lines.
If the Ergo really isn't very comfortable, I'd see if you can build up your tolerance before you go if you have time and see if you can adjust it to make it more comfortable. I find that it's most comfortable if I wear them as high as I can. If you wear them too low, it puts strain on my shoulders and lower back. I'm not really a very strong person and practically never work out, but I've been able to wear my young toddlers 4-5 hours/day for many days in a row when we've gone to WDW. The last time we went, we had an 11 month old, and we stayed for a month. We went to the parks for more than 20 of the days we were there, and I wore the baby most of that time, but my back never hurt. Unless you have a problem with your back, you can probably adjust the Ergo and find a way that it's comfortable to you. If you do have a back problem, they come in sizes that fit husbands, too!