I know of people who are successful at agility from their wheelchair. It's all a matter of training.
You will be more successful if you work with a trainer who is comfortable/familiar with training dogs to work at a distance. A lot of AKC trainers that I know are all about running with the dog and working with nothing but front crosses. This is very difficult for those with fast dogs and/or limited physical abilities.
My big dog, Luke, works well at a distance and I'm pretty spoiled by him. I still run courses with him, but there are times when I can just jog or even stand still while he goes out to do, for example, a pinwheel without me. I always take the shortest path with him and am often able to send him out while I cut across to get ahead of him.
Kaiser is green, small and fast. I can't keep up with him to save my life -- from the beginning, I've worked with him on distance and obstacle independence. It's getting better, but I still am very winded after working with him because I have to run much, much faster with him than I do with Luke. I know that I hold him back at times, but I just adjust my training to try to fix that (for example, by working on sending him ahead of me, not just our lateral distance).
If you have a slower dog, you will have a much easier time of it. Usually the beginners classes don't ask all that much of you and I'm sure you'd be fine. Give a go!

Luke -- Lab/Shepherd (1-1-04) Elite Versatility, O-EAC, ECC, O-EJC, S-TN-E, O-TG-E, WV-E, HP-E
Kaiser (CH'PR' Alasco's Kvichak) -- Alaskan Klee Kai (12-30-07) TBAD, TG1
